You nailed it.
Even when folks have done most of the content, as long as they have characters/spheres or something to work on/work toward, they'll keep busy and keep playing.......but if they get one too many bum summons in a row where they get nothing at all new to work on.....they run out of things to aspire to have/do.....and they find other things to do with their time.
Ishad is further down the road than I am, but even I have all my 7-stars sphere'd (not all imp'd).....but even I already have a 7-star Fully Imp'd that I'd consider pretty much obsolete (read: very much outdated, not useless) at this point: a fully imp'd Anima Diana, I really thought she'd last longer than she did....but now she is just regulated to 2nd/3rd or even 4th squad duties on Trials/GQ's etc.
Probably the biggest reason Alim keeps putting out new units like gangbusters is to keep people active and give them lots of new shit they can be doing.....Gumi would do well to follow them on the God Gate.....although I admit, I'm holding my breath a little, because it isn't exactly "content", and I don't have 100% faith that Gumi will implement it either.....but they SHOULD.
Looking into the future, I can see that my time here would definitely be shorter if they don't.
Heh, I've steadily lost interest over the past month or so. Haven't tried Xie Jing or the new trial, haven't tried any Grand Quests past Seria's, only tried the new GGC once. Hell, I haven't even played today aside from logging in. Whether I need units or not, their acquisition is the bedrock that underlies the fun of these games. That's why I've argued (to all appearances unsuccessfully here) that they need to release MORE units - not fewer - in addition to a steady stream of content.
And I don't mean MORE truly meta units like Deimos, Tridon and Zenia that leaves gem-starved players feeling well and truly fucked. I just mean MORE units - new mechanics, maybe more than half a dozen fucking mitigators, sensible updates to obsolete units, etc. And for the love of God, reddit BF fanatics, stop dropping hundreds of dollars on new batches without rateups.
I'll preface the following by conceding that no current content (that I've completed anyway, and I've completed GQ, which Ishad I think said is the hardest in Global) requires a squad of entirely meta units. Still, something shitty's going on when everybody completing current content (save Ishad - and kudos btw for undermining my assertion that Zenia is required for FG) is using the same fucking units plus a mitigator, with 1-3 of 6 slots filling other roles. The metric of success of late - subtracting cases of admirable tenacity from people who didn't blow their load on global units - seems to be a question of how many specific units, rather than specific roles, you bring to the table.
I haven't been playing seriously for long, but I do seem to recall people coaching new players on the necessity of sparkers, mitigators, crit buffers, gauge fillers in any squad. Now it's a matter of how much you can fill up your team with Zenia, Bestie, Tridon and Deimos.
There's also entirely too much Reis to Glyph going on and not enough Kikuri to Medina going on. I get it, clear upgrades are more fun, but not when kings are usurped a batch away from their introduction. On the other hand, I greatly enjoyed Medina coming out so close to Kikuri because they fill the same role but slightly outperform one another on certain tasks - this was key to my FG team. Two similar units with slightly differing toolsets accomplishes two things: first, they're both useful - even to people that have both, and second, your chances of getting a unit that does more or less what you want is essentially doubled.
If I didn't have a Kikuri or a Medina - and possibly even if I had one but not the other - I'd love to summon during a rateup that had the two of them. But this shit I was stupid enough to summon for two days ago? It had one unit about to be truly obsolete - Colt -, one that's been relegated to niche use - Kira -, another slightly niche but still pretty good - Dion -, and a current king soon to be robbed of his crown - Reis. Fuuuuuuck that.