They're one of the most common teams to cycle through and sell good players for teams not quite at their level, same as PSG. Because every couple of summers they will have a year where they sign 3 or 4 first teamers. They keep the ones they like best (they arent always the best, they just like them the best. Others might be just as good but their have a style preference) and then teams who are 2nd and 3rd best in their divisions can pick up good players from Bayern sometimes at a cut price because if its one of those summers they need to clear out some of the squad.
Palhinha has been a good loan for Spurs, Tilman did well enough they bought him back, Gravenberch, Sabitzer has been a useful signing for 2nd placed Dortmund, Lewandowski to Barcelona, Chris Richards has been good for Palace - all in the last 4 years and discounting Mazraoui who has obviously been worth the price as hes with us.
Some of their sales havent done the business elsewhere and De Ligt is certainly questionable to this point. Especially now he has this current injury and with Maguire doing just as well in his absence. But its not because Bayern dont let go of useful players. They let go of plenty