Actually, hentai must be hurting as well, if only because if it was doing well, MediaBlasters should be doing well (which we know they aren't,) and the fact that again, it's something the audience is prone to pirate over purchase is probably a big piece of that (especially since the internet has seemingly changed the minRABet behind pornography as whole - it's not about rewatch, it's about constantly being new and pushing into areas.)
Even then, hentai is never going to be really judged if the plot sucks or often times even if the animation is wack. There you are going for one thing only, which makes it an easier to handle thing than a show that's trying to balance elements against each other (as most fanservice series attempt - sometimes that's action, sometime's it's drama, sometimes it's comedy and surprisingly you'll see a lot of all 3 in play.)
And even then, there is crowd that wants pure porn and a crowd that'd like something a bit more subtle, and while there is crossover, one is certainly larger and more difficult to please than the other.
See, I'd go the opposite, or at the very least, I'd start with the emotional conflict that exists within the male lead - start with a cold open on him telling his father that one day he'd like to marry his sisters, with him then being told he can't do that because even though he's not blood related to them, Japan does not allow polygamy. Ground the drama and thus sell the characters first, then you can get away with more service. Plus, you go from a show that people will only watch for the service, to something that gets actually closer to Ditama's full intent, which is to have the service as one of various hooks to get people into the story. I've read the manga too, and like Mahoromatic, I can see he's working toward something bigger, but the show undermines that by adapting him verbatim.
Thus we go full circle to if it's a chore, it's not a good show, no matter it's trying to accomplish, but when it's trying to cover for that with fanservice, that's worse.