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.