From a content standpoint, I think there's a good 2-3 "chapters" more possible for this story. However, one of the mistakes the devs may have made is calling them chapters in the first place, because it opens up the game/dev to accusations of milking or slow progress. Had the existing "chapters" been called "sequels" we'd be in the same boat as now, but the counter argument to "milking it" would be, "What? This is the fourth game...." Five years of development is a long time for *a* game—in the context of F95 at least—but for 4 games? Not so much. Sure, this is a semantic argument, but comparatively, look at games like Something Unlimited, which has no end in sight, yet very little actual content that isn't repetitive to the point where it actually makes a superheroine porn game with good art into a slog. That's not even mentioning some of the more infamous projects out there.... ::cough, cough:: Innocent Witches ::cough, cough::
We're also in an era of long-running entertainment IPs. I'll not bother listing studios/franchises, but 5 years for sequels (even called chapters) isn't unusual in the broader context.
All that said, I don't know how much of the dev's support comes from DMD anymore. Clearly it launched made them into big fish in this small pond pretty quickly, but they might have moved on in ways not really visible from outside. I think they could carry on with this project more or less indefinitely, though.
