You're completely right about the difference between playing "in the sandbox" vs. "on the rails".
I think it comes down to playstyle. Some people want more of a sandbox. Some people want more in the way of rails.
Anytime something in the game arises that might provide limits or challenges to what sims can do, you see people respond in both directions. The more sandbox-y players don't like to have too many limits on what's possible for them to do in the game. The more rails-y players hate it when too many challenges get "nerfed" away.
Scenarios are an example of gameplay that adds rails to the game.
So are things like legacy challenges that people choose to take on.
I don't think there's anything illegitimate, or "not in the spirit of the game," about either playstyle. I do think it's nice to be able to adjust the game a bit in one's own preferred direction, within reason. Like choosing one's own DLC. Or playing scenarios, or not. Or choosing what parts of the game to use more and what parts to use less.
If you are more of a sandbox player, then maybe you might be happier not playing active careers at all.
Or you could modify the career lots and give your characters other things to do besides just tick off the tasks on the list over and over again. After all, unless they are super-driven career-track-minded people who want promotions all the time, they probably aren't 100% focused on their job tasks the whole time they're at work. The performance meter is forgiving enough that you have room to ignore some tasks and do other things, and you'll still get promoted now and then even if you only get Silver instead of Gold at work for example. I tend to play my active high school students this way.
Or, you can delve more into retail ownership gameplay, or alien gameplay, which are both terrific sandbox-oriented things that came with GTW.
