Look, I hope this game keeps going for a while; I'm enjoying it.
And of course everything is pure guesswork here.
However...there's no denying the population took a huge plunge if you judge by guild activity. I'm in a top 5 guild, and today's competition was 2 1-person guilds and 1 19 person guild that did absolutely nothing all day.
And I'll grant you that FKG's compensation for the closing games pales in comparison to senpros, and Senpro has gotten much more player friendly in the last month or two. If you were playing in February, though, you'd remember that any gacha that gave you a 5* on a 10-pull was paid magatamas only. F2P all you could do was roll at the 2% rate and hope for the best.
The other difference is that in FKG you can generally grind your way through everything with 4*s and event 5*s (and those can be readily obtained by even new players). In SenPro, there's a lot of tougher content that isn't available to you if you don't have strong senki, and you're at a significant disadvantage in GvG. These are all things that turn off a lot of players.
As far as why a game you think is bad gets to stay...dev opinions and your (or my) taste don't have to align at all. If HH is bringing in money and has players, and dev support, it will probably stay.