To my understanding, Sugar intended to end the show with the movie, but either during production or pre-production, CN told her they were getting one MORE season greenlit on top of everything--one she didn't have concrete plans for.
Now, I like some of Future--and I'm pretty sure the stuff with Jasper and Gregg was shit they had planned for the main series but didn't have time for, so they brought their A-game with that. The idea on paper wasn't *bad*: as Steven goes through a full out breakdown over the course of the season, all the episodes are basically vignette send offs to the rest of the cast to bring everything to a close.
Except... it still felt rather condensed, and... rather predictable. It took /co/ all of 3 minutes to figure out Steven was going to leave Beach City at the end, based on the credits animations, you could tell the whole time Steven was going to get worse even amid moments of hopeful improvements.
Knowing from the start how it ends makes the ending feel more hollow and unsatisfying. Sure it was cool to see how the other characters were moving on, and Steven not being able to move on led to his downfall but the season could have used a couple re-writes and hiding the endgame in order to save it.
I can only imagine if we DIDN'T all guess Steven was going to leave Beach City, it would have been a much bigger impact, but that's just me.