For the record, I think it can work. For example, Simon of The Last Sovereign is a competent, mature soldier who never outshines his cast. Of course, as an rpg, Last Sovereign can do rotating perspective which gives it a leg up but I think it's still doable in the AVN format.
it probably is, my thing with the avn format is it's really hard to have high quality animation when there's a lot of content. for example, i tried playing a game called headmaster (you can find it in itch.io as well) and while the dialogue is very well written and there's a lot of content for it, the animation is very cheaply made. for a mostly solo (afaik) developer like Caribdis, high quality animation and the amount of writing it would take for all the characters to be able to shine that much is probably more work than is feasible without jeopardizing timely releases or quality oversight. Ideally of course, i'd like it to be possible, but i dont see how it would be done well.
This is a harem game. If MC didn't have at least a little dog in him, he'd have zero chance with Nancy, Alex, Penelope, or Dahlia. He wouldn't get Nova to share long term, either. Annie and Luna, maybe, but that is based only on scenes already given; if Luna could not do what she does, he'd have to choose one or the the other.
Headmaster is more difficult to write than you think. I believe (but am not certain) that the story arcs of the girls and staff are all outlined and plotted, but the order of development is, to some extent, voted on by Patrons. With 23 named students and a half-dozen scene-available staff, events in the past for one that haven't happened yet for another being years apart on release can make updates very difficult. As for the animation, I think it was started around the same time as Once in a Lifetime.
The writing is everything. Eternum and Once in a Lifetime have a more international gamer culture appeal. The Headmaster can be difficult for non-English speakers, some exchanges even for fluent non-native speakers. But for all three, the writing is excellent. In most other games, it ... isn't. Those games are still enjoyable, but I sometimes lose my headspace with some of the more egregious typos. It shouldn't matter, I guess; I am just an old man yelling at clouds. Even Ripples, which I love, makes me flinch with all the 'anyways' and sentences ending in 'at.' And that manner of speaking is accurate for the (current era) characters depicted.