Theres plenty of good reasons to end it and 40% more monster girls that already exist in the game is quite a lot.
Reasons to end FOBS
1. The engine hes working on is archaic and no longer supported.
This is probably the biggest reason, Zell is always complaining about issues surrounding the engine, it's limitations, it's bugs and the fact that it is no longer supported by the creator, Zell has had to do more on the programmer side of the game simply because the engine is so flawed, if Zell ends the game and creates another similar one on a different engine or platform, we'll all be better off. Theres also the issue of higher quality sprites, compare the resolution quality of spites from Zell and the ones from Kyrieru, there are more pixels overall with a greater level of detail. I don't know if the engine is directly the reason for this but it certainly can't help.
2. Zell is running out of ideas.
This is a distinct possibility, after all, there are only so many ways you can depict a poor dude getting fucked. We have so many sex sprites but the flavor is really what makes them unique, we're not talking about sex positions here, which is all well and good, we're talking about how a monster girl with her specific monster girl toolkit uses it to rape our hero, i think Zell envisions this better than anyone does but when you have two harpys in the same update and they both swoop down and fuck you, you are probably reaching your wit's end, this and the fact that it's been the same protagonist this entire time really limits your ability for creative potential.
3. Zell just wants to do something new.
Zell is a very talented creator, but it is unlikely that his own Hentai tastes are limited to Reverse Rape. Kyrieru's first game was a traditional ACT game with a female protag with all kinds of monsters and tentacles which preyed upon her, for all we know, Zell wants to change it up and give his own take on that genre, furthermore, there are so many different types of games now, FOBS is a traditional platformer, but then you have games like this
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which is a run or rape type of game, it's still monster girls, it's still reverse rape, but it's a different gameplay altogether.
If one thing is clear it's that Zell is a very talented developer and artist whose star has risen tremendously while hes been working on FOBS and not just within the Hentai games scene either, DLsites seems to place many Reverse-Rape oriented games on his website as advertising and i think hes big enough now that any actual game put on DLsite would sell incredibly well. I don't think Zell is going to disappear entirely when FOBS is done, it may simply necessary for the evolution of his craft.