Maybe I've misunderstand you but I don't think they must sell (out of financial reasons) to "survive" like most of other (smaller than BVB) clubs have to do.
I think they just prefer to sell players like Götze and Hummels (they were both sold for their buy-out clause IIRC so not much they could do to keep them anyway) that don't want to be there anymore? (which is good IMO because there is nothing worse for a team chemistry than keeping an unhappy player).
Mkhitaryan and Gündogan also both wanted to leave and Lewa left on a free transfer (because they prefered to keep him for his last season instead of selling him a summer before; don't know how smart that was financially but if anything this shows that they don't always have to sell their best players when they don't want). Other than that I don't remember a case where they had to sell an important player over the last 4 or 5 seasons.
While it's true they can't financially compete with Barça, Madrid, PSG, Bayern and top EPL clubs I don't think they really have to sell their best players until they can keep them happy. But if some club will come with a crazy offer like 70 or 80M for Aubameyang of course they'll likely sell him. That's what any financially responsible team outside of the mentioned top 7/8 teams in the world who can afford to reject those kind of offers for their best players would do. And while Auba is a great player he'll also turn 28 this year and his game is based on speed so who knows what will happen with him in 2-3 years when he'll probably physically decline and lose that speed.
For other young players they bought over the last year and half (or their homegrown players like Pulisic and Passlack) I believe they can keep most of them together if they'll develop well individually and as a team so they could possibly challenge Bayern in BL and maybe with some luck even be a serious contender in CL in 2-3 years. But again, if some club will come with 50 or 70M offer for Dembele or Pulisic it will be hard to turn them down. But whenever they'll sell it will be most likely under their conditions.
