Why so angry…?

I usually try to be positive, but even I can give a few examples/reasons:

1. The myriad of bugs, new and old. There are bugs that have been present for a few months at least and have yet to be fixed. And every other update somehow brings at least two new ones. Look at the immortal GG and oFulcrum bugs. They showed up out of nowhere, and it took months until they were fixed. Last week's update took care of TIE's gaining marked due to Scythe under Chimera, now they don't get marked with Executrix, even though Scythe's unique specifically mentions that they should be getting marked under with Executrix. Or to go further back, that Conquest where we had GamGuards and the Rancor as bosses in S1. They were bugged (or overtuned, I can't remember which), and we were lucky enough that CG managed to fix it during the first few days. But the fact that it had been bugged at all, and in the very first sector. The enemy Levi stunning not one but two of your ships whenever it calls a reinforcement after having used its Ult ability.
2. Lack of actual communication and reasoning on major changes. Just look at the AB fiasco. They start adding new ABs (DoF had at least been announced in the last Road Ahead, I think) and announce that the old ones would be gradually rotated out. The problem? The latter announcement was made two weeks AFTER the players noticed that there was something wrong. The recent fleet changes. P1 came out of nowhere, it turned out to be too strong, not only against Prof, but also against Levi. What did they do? They reworked Dagger and now Levi mirrors are a coin-flip dependent RNG fest where even if you go first you're at risk of timing out, P1 still does well against Levi. No news on whether there will be more changes or at least a post explaining why those specific changes were agreed on.
3. Replacing old content with new events, therefore invalidating previous roster investment. What is happening with ABs is a very good example. Most of the old ABs have been around for years and are very useful for players of all stages - new, mid, late. Players have been and even now are gearing/relicing teams for those events. All of a sudden, we are told they are going to be gradually replaced with new ABs, namely ones that require the newest toons, have much stricter gear/relic requirements, have much higher RNG regarding what kind of rewards you get (compare the more static rewards from the old ABs with the one-or-the-other rewards in the new ones). The new raid system, you have raids that require you to relic up specific, and in many cases garbage, toons and use specific strategies, only for a new raid to show up not even a full year later and require a whole new set of toons. Look at the Lumi team for the Naboo raid, I doubt many people decided to take her to relic levels for reasons other than the raid.
4. Constantly increasing requirements for events. RotE requires toons to be at least R5 just for the first wave and we go all the way up to R9. More and more JG toons require R5 minimum (Starkiller, Aphra, JJB, GI), some of them R7 minimum (Bo, Baylan). GL Ahsoka requires 3 R9s, multiple R7s and only 2-3 R5s, nothing below that. And even if you are at the minimum level, the event itself is so overtuned for that level that you have to go much further, which leads to the next issue.
5. Too much RNG and overtuned enemies in events. The Naboo raid is a wonderful example. The enemies are massively OP in terms of stats and especially modifiers, the player modifiers are almost meaningless. You can get 1.2M in one run and barely half of that the next week with the same team forcing you to restart AT LEAST once more, which is especially annoying if it happens several minutes into the battle. You can be at 1/4 of the max score for a tier while the enemy team is at 70% enrage and keeps getting constant bonus turns with a DI DDK. With the Krayt raid, I could go in with mostly G10-11 Jawas and still get the max score for that tier, simply because the player bonuses were enough to balance things out. Right now, the bonuses in the Naboo raid are pathetic in anything below the R8-9 tiers. And I've seen complaints that even in those tiers, it's still a pain. To use the example you gave, with Lumi, yes that team does well in the raid, but it is very dependent on having both HoT and PotUp at the same time, and especially on your team counter attacking enough during that time. Yet, if for some reason they rarely counter attack or if the Command Droid keeps getting bonus turns and dispelling your entire team, you're in trouble. Another example is the Geonosis and to a lesser extent the Mustafar fleet missions in RotE.
6. Massive time sink. Look at the weeks where you have TB, Conquest and Raids along with minor events like SR II, EE, UJ, etc., running all at the same time. Then factor in the RNG-induced multiple attempts, like with the Naboo raid.
7. OP or bugged DCs. I for one like the idea behind DCs, boosting older and less used toons/teams to higher levels. It's the execution that leaves a lot to be desired. I don't even mind that some of the mechanics are incredibly strong. But every time something like that is released, the entire playerbase immediately starts putting it in GL teams, which are already strong enough by themselves. DCs like the ones for Padme and Holdo on GL teams, making DC bonuses specifically for GLs (Leia, JMK, Rey), or having ridiculously high stats like Evasion and Defense on GL teams. To say nothing about whether or not the DC is working properly. For example, the Inq DC a few months ago, the one with the Lv6 assist on Purge mechanic. The way it had been worded, it was supposed to call only one ally to assist at a time, yet every time I went up against an Inq team with that DC, the entire team basically started swarming me every time someone used an ability.
8. Specifically designed first-run DCs. POW, GMs, Baylan, am I missing anyone else. The only way you can get those DCs is if you complete the event during the first run. And the only way you can do that is if you spend a massive amount of crystals or actual money to get the one toon to relics (POW and GMs) or to get several toons to relics so you can unlock the event and get the actual toon (Baylan). The latter case I can understand, but the marquee DC thing. Huge middle finger, especially with how ridiculously strong the Great Mothers DC is. I agree with what you said about being careful with your spending (I'm completely F2P btw) and not expecting every toon to be the "new apex predator", but I can also understand players being frustrated because of how the toon had been advertised or due to how high its requirements are (Baylan, for example, requires 5 R7 toons, with 4/5 of them being Marquee toons that were released 2-3 months ago, and 2 of them not even being node farmable yet).
9. Constant lagging with some events or teams, plus long animations. One could argue the lagging falls under the category of bugs, but I'm putting it as a separate issue. Tuskens and Gungans (albeit not as hideous now as it was the initial weeks, for me at least), AA+VA disk combo in Conquest (I use an ability and suddenly the game freezes for at least 15 seconds until it resumes, heaven forbid I use LV's AoE). And the animations on most of the new toons take way too much time even on 4x speed. The enemy team itself may be annoying but still beatable, yet you end up timing out simply because of how much time is spent on the animations.

And those are just the ones that immediately come to mind.

The game is unquestionably in a better place than it was. That trend won't continue if they continue to devalue prior investments.

People like the old assault battles because they were a sure thing - they were clockwork - you invest in a thing - you get rewards from that thing that you can count on. We all wanted more assault battles for that reason and that reason alone. If at any point CG said, "hey guys how would you feel about some new assault battles that replace the old ones" - not a single person would have been cool with that. 

The Dagger changes attempted to fix a problem that the community already solved a long time ago - a problem that we begged them to fix a long time ago. The problem with Levi has always been the mirror match and how miserable that battle is with Tie Dagger. From day one this was highlighted and underscored as a problem and they did nothing to address it. There are so many ways they could have handled this and improved the experience of using Dagger without making the entire meta a frustrating coin flip that delivers either an instant-quit or a 5 minute ride on the struggle bus.  Never mind all the lost investment in Sith Assassin to compensate for their own inability to respond to community feedback. We won't even talk about all the people that prioritized the Chimera fleet as a counter - another instance of fixing something that was broken that people had come to rely on and thus, removing value from their investments. 

Even if they got rid of fleet arena, this will persist in being a problem in GAC. It certainly won't be every match - but matches WILL be decided by the Levi coin flip. That's gonna feel bad and it's going to be bad for the game. 

This treadmill they're putting us on started with datacrons, ramped up with rotating raids, is ramping up more with rotating assault battles, and the changes to the fleet meta have only added insult to injury. 

I totally understand people wanting to say positive things to buck the trend of negativity. I like the game too or I wouldn't be here - but the game that I fell in love with years ago is not the game we're playing today - and the game we're playing today won't be the game we're playing in two years if things continue in this direction. CG seems to be buying in to the idea that people want to be chasing something all the time - which isn't necessarily wrong - but if I never gain ground then the miles behind me don't matter - and if the miles behind me don't matter then why would I keep running?

edits for clarity in bold