The "one true ending" after the purity routes is 100% confirmed won't be a Harem Ending. So I tend to believe that if Akira won't have all the girls he won't have any. Don't think Sel is going to play favourites here and say Sensei's "true love" was Maya/Niki or whoever. What I'm kind of expecting is a bittersweet ending, the cycle ends and the girls are ready to move on with their lives and leave their traumas behind. And a redeemed (kind of) Sensei being gone for good, since some girls don't seem able to carry on with their own lives as long as he is still there in any shape or form. Maybe a couple of them (Maya, Ami) won't still be able to and will try to create a new "dream world" and that's how it ends. I'd rather have them to not do that and learn to live without him but that probably won't be the case.
Anyway, the existence of multiple universes is canon on this game, so if you don't like the "true ending" whatever it is, you can still pick the ending of your favorite girl and take it as your own ending and that will be okay I guess.