It depends immensely on the game's theme and the mechanics' execution.
If you have some shitty combat minigame... I'm just going to look at the gallery. Sorry homie. Very few minigames are fun.
Something like Monster Girl Dreams, where you have an actual battle system, I can absolutely get addicted. CoC1 was pretty good for that too. If it's genuinely fun, I may very well enjoy it more than the lewd stuff. My inner JRPG/Disgaea fanatic comes out and I just want to make the most fun, hopefully-effective build possible.
RPGM CAN do the trick... rarely... but most battle systems are either the bare minimum there, or customized into something even less fun somehow. I won't instantly deny an RPGM game, but even better-known ones like TLS I bounced off of from hating the barebones combat so much.
Also, rewatching an event 20 times to get +2 strength isn't real gameplay, much less an interesting RPG system.
Trying to make a fun combat engine/system for my game taught me very quickly why most people don't bother, though. It's an INSANE amount of work, especially if you want a variety of playstyles/approaches to combat. Frankly, mine isn't even particularly creative in most regards.
That's all *without* worrying about balance, which is another endless struggle, especially as you learn more and try to make more interesting mechanics that may end up significantly stronger than old ones.
To address your question directly OP, there are a lot of people who are just there for titties, and it's even more pronounced on some sites like gamcore. Adding a full RPG battle system is NOT an effective use of dev time from a financial standpoint. I've been fortunate to build a community that appreciates it, but if I was looking to make a quick buck on Patreon it would have been drastically smarter to just focus on CGs and lewd scenes right from the start instead of wasting a thousand-plus hours on a combat system.
That said, I would absolutely play something like a lewd Mass Effect, and I think with enough production quality it could be extremely successful. Unfortunately, the adult game industry in the west is a dumpster fire compared even to Japan, much less the 'actual' games industry. So... I don't think that's happening anytime soon.