Actually, if you think about it, Barcelona can get Robert Lewandowski because why does this Barcelona need a goal-hungry striker like Lewandowski but this Barcelona now has Pierre Emerick Aubameyang, it would be a shame if for example bringing in Robert Lewandowski if it was just a cover for Aubameyang or vice versa, So if you want to bring in Robert Lewandowski, you have to sell the existing striker
I don't think it's like that, it's true that currently Barcelona have a lot of strikers there and one of them is Auba and some have potential like Ferran Torres or Adama but on the other hand they also still need goal-hungry strikers like Lewa but like you said when you want to sign players it's clear that there are definitely some who will be sacrificed and there have been some that have entered the radar of other clubs such as Dembele with PSG or some issues regarding Braithwaite with several EPL clubs.