That sucks man, it's no fun having currency taken away, but I'm sorry to say I think the move of crystals from arena to GAC was intended to address your situation exactly. The way arena used to be, the people who played at the highest percentile of performance (or wallet depth) were able to hoard A LOT of crystals with little effort. The forums are full of old posts with players who in the past shared your exact same experience, which usually involved them telling F2P players who complained about the game's accessibility or pay walls that they "have 10,000+ crystals and that's just from arena so this part of the game you're complaining about isn't that hard."
Unless you meticulously planned your spending, people who played anything lower than the top basically only ever had enough crystals to refresh energy a couple of times each day and not be lower than they were the day before (meaning your primary crystal sources were likely just guild events). For example, in the old arena I consistently ranked anywhere between 100-150 in my shard. Even with the currency I got from playing with a well-organized and decently competitive guild, I usually only ever earned enough crystals each day to max out on raid tickets with energy refreshes should I need to, and still see only a small net growth in crystals each day. In the new system, as a mid-tier player (I came in at Chromium I), I definitely noticed a large uptick in my crystals. Whereas in the past I'd usually sit between 500-1500 crystals, I now sit anywhere between 2000-3000. This is all to say that in the old system, basically no one other than top performers were seeing consistent and large growth in their crystals, and I imagine that presented a few problems for CG.
From a marketing standpoint, CG knows that it's the top tier players who are spending far more than others, and with the daily earnings CG has taken away from those players with the change, there is now not only more incentive for them to spend currency to get the edge on other top players, but also the pressure of encroaching lower tier players who have had their currency boosted by the change. In the old system, CG was basically giving the most money away to the people they most relied on to spend money on the game, which is obviously counterintuitive for a profit-driven mobile game. The new system redistributes that free money so that on average it might encourage more spending across the board.
From a gameplay and engagement standpoint, it makes it almost impossible to earn a lot of crystals without decent PvP skill as you've described. Now your earnings are determined less by your ability to spend currency to earn great characters for that one meta team or how late in the day you get your battle in, and more by your knowledge of the game across multiple meta teams and your ability to strategize over a longer period of time. There was also the matter of many players complaining that there was nothing to do in the game and being demoralized with the lack of incentive for engagement. With the new system, CG has changed the playing field so that their top players have to spend and know more to stay competitive, their lower players are put in a position where they can see some real benefit from spending more if they know the game well enough, and all players have an incentive to engage with the game more. CG has more or less removed any avenues towards earning a lot of crystals passively as people could in the past, especially for those who aren't very good at PvP.
TL;DR The loss in crystals that you're seeing is intentional and targeted for players in your situation who were earning a lot of crystals passively in the old system even if they didn't have very good knowledge of the game. CG knows that their top-tier players are the ones who spent the most on the game, and giving them a lot of crystals for free to the point where they didn't needed to worry about spending is counterintuitive if you're a profit-driven game. All I can recommend is improving your strategy and upping your engagement with the game.