There are quite many options at Bayern Munich actually when we check their squad. Let's say Sommer didn't leave then there are 4 goalkepeers available as alternatives to Neuer. One of them is a really young one and let's say we kept him outside. Ulreich, Nübel and Sommer are the ones left.
If Neuer doesn't start playing right away then Bayern Munich can actually reassure Sommer about being the main goalkeeper here. In that case Sommer wouldn't leave and the problem would be solved. Otherwise Nübel can be a decent choice maybe until Neuer gets ready to play.
I still think that Sommer may leave, there are talks about him leaving and that makes sense a lot to me, I know that it is going to be a bit of a tough deal but at the end of the day it still can happen. I believe that we are talking about enough backup goalkeepers that if they lose just one of them then it is not going to be a big deal.
Of course Sommer would rather be the main goalkeeper there, but we know Neuer will be back and looks like he is going to be ready by the early season games, so it is not like there is a chance Sommer will keep that place. This is why it is better for him to leave and make a bigger impact somewhere else. I understand the logic, but Bayern should be understanding of his desires to leave as well.