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.
I mean, if De Gea is so that great, many teams should be interested to sign him after Manchester United not want to extend his contract, isn't? but the reality there's no big team is seriously want to sign him, Bayern Munich is a big team and they're need to find a good goalkeeper.
I think it's because of De Gea want a high salary and no team can afford it except Saudi Arabian teams.
I think it has to do with his age, because now many big clubs are looking at younger players, and when they have a goalkeeper who is senior enough it is a goalkeeper who has been at the club for a long time. David d Gea is not that great, but his ability is still reliable. Manchester United did something hasty, so they did not extend his contract. The fact is that now they have a goalkeper who has no better performance than David d Gea.