I didn't find it logical for them to sell Sommer either. The goalkeeper they are looking for must be at Bayern's standards. I think De Gea has these characteristics to a great extent. He can easily save Bayern for at least 2 years. They can easily play goalkeeper until the age of 38-39. I don't think expectations from Neuer's performance should be high.
If you look at the experience that De Gea already has, it is still quite feasible for a team as big as Bayern Munich to attract him into their squad. Because De Gea is not that old, it is still possible for him to be trusted again as the best goalkeeper in a team that has good dominance in the Bundesliga. And if Sommer can still make a good contribution to Bayern Munich, he doesn't need to be sold either, but for now I think it is also difficult for Bayern Munich to make a choice on this matter.