What if he wins scudetto's in a row and has good results in UCL
It depends on what you consider as "good results". IMO there's a pretty slim chance of him achieving it but there's always a chance, yeah. For me he has to show improvement as a coach if he wants to stay longer, not only looking at the board and waiting for better players joining in every Summer. We already have a very decent squad which underachieved in Europe. When you look at what Klopp or Simeone are given to work with you realize it's not just about signing better players every Summer.
As for results I want him to get a scudetto/scudetti but more importantly he has to push this squad to its limits in Europe. If he achieves semis during his time here (I guess it'd be about 2-3 years of trust) I'd be able to give him more. Otherwise we'll have to look for a coach that will be able to make this squad overachieve in Europe for once instead of constant underachieving or doing 'ok'. If we never make that next step and "overachieve" (at least) for once it'll be a vicious circle. We'll turn into Arsenal of the CL and I don't think that's a dream for our board or any of us - fans.
I'm not being impatient with Allegri but I'm trying to take a step back and looking at a bigger picture. Conte has done all the wet work for us and I still have my doubts if Allegri can make us go any further. Latest formation change, results and the way we played made me somewhat hopeful while before that he was frustrating from my POV. The time will tell.