It's becoming increasingly obvious that this season will be, to use a generous adjective, a transformative one. At the end of which we will again have to sell whoever happens to be the easiest player to pawn off.
Once you start seeing things that way, there are no benefits to keeping Handanovic as a starter. This has nothing to do with his mistakes, keeping him as #1 at this point is merely indulging him as a thank-you to his years of service and/or captaincy. I also disagree that people have negative feelings towards Handanovic solely because he is some kind of remnant of the banter era.
I would let Onana play from now onward. Either he proves to be a good match and becomes our GK for the next couple of years (best case scenario), or he has a just good enough performance that he boosts his price and we sell him next year and get another GK (if we have to sell someone, this is also a very acceptable outcome). Worst case, he doesn't gel and is a poor match. Well, we still have Handanovic who can ride out the end of the season and get the farewell he and some others here want.
We should be more pragmatic honestly. Toldo and Cesar had enough grace to step down when the decline was clear.