The problem about teams in the Bundesliga is that they are not even worried about selling their players to Bayern Munich. Frankly speaking, I don't blame them. Bayern Munich is the only relatively rich team club in the league and they always have the resources to sign whoever they want.
Another problem is that other teams in the league lack squad depth and Bayern has a lot of those despite have more quality than every other team. For the foreseeable future, Bayern will continue to win that league.
Financially Bayern is rich and that part is true but other German teams are not exactly poor neither. I mean look at Dortmund, they sold a few players in the last few seasons that would dwarf what Bayern sold, and yet even with that they failed to build a great team with all that money. This is a proof that it's beyond just transfers and buying players from bundesliga or anything else.
It is about buying players who are good for cheap. Look at how cheap they got Mane for, many teams pay huge sums for players that talented, and Bayern paid peanuts for him. Losing Lewandowski was a big hit for them, but they recovered very easily and got a new star.