a better chance of having both QBs come back next year if both of them think they have a chance to play. I think Kelly's story of using each QB because they have different skill sets is nothing more than a cover story, an excuse.
Recall Kelly saying he had a conversation with Golson and Golson agreed to take the "journey." I think Kelly wanted to make sure that Golson was committed to staying or he wasn't going to waste any time on him in practice just for the sake of one bowl game. Golson's agreement to stay another year is good in that it will leave Kelly with an experienced QB and maybe, just maybe, Golson has the competitive determination to get his mind focused and can get his head to do what it needs to do. Golson, I think, technically is the better QB, i.e. he throws more accurate passes, but his mind has a certain lack of confidence which causes him to not throw the pass on time. If he can somehow remove that doubt, he could be a very good QB.
As for Zaire, he still has enough eligibility left that he could leave and still have a couple years left to play somewhere else. Obviously Kelly doesn't want that. I think Zaire has had meaningful playing time now, even if it wasn't very much, so I can see that he would be offended if Golson was put back as full time QB. Zaire is not as good a passer as Golson but he is more reliable in all phases of the game. He is worthy of playing. If Golson can't fix his confidence problem or gets injured, Zaire is an indispensable asset to the team.
The worst thing that could happen to Kelly is for Zaire to transfer and Golson to get hurt. That would leave Kelly with a completely inexperienced QB once again. The ebb and flow of the QB drama has Kelly as frustrated as losing 5 players before the season started. I know Kelly doesn't like to play two QBs, but this might be the only way he can keep both for next year.