It really is a myth. Lewandowski essentially ran his contract down and refusing to get sold elsewhere, Goetze came through release clause and Hummels was in his last year of contract.
Contrary to very popular assumption but Dortmund isn't actually a selling club.
Go back the past 10 years, all the "stars" they sold fulfilled one of the following criterias
- release clause got triggered (only Goetze)
- had =<1 year left of contract and didn't want to renew
- literally went AWOL, insulted the whole club and striked themselves out.
There is no exception to these 3.
Name any Dortmund sale with hype in the 2010s, and he will have one of these 3 criterias fulfilled, Dortmund was never left with any actual choice in these sagas about their star players.
This falsely gave them the reputation of having some sort of "business model" to buy cheap and sell expensive, that is nonsense. The thing is, they are indeed
genuinely attempting to build a great squad since years, but then some dudes contract runs on 1 year left and all of Europes top clubs show up and poach them with insane contract offers, promises of titles yadda yiddi yadda.
People are giving them a lot of shit, but what are they supposed to do?
They are actively trying with everything they have to create a squad with longetivity that breaks Bayern monopoly and challenges internationally. But all the players refuse contract renewals because they do not want to wait till Dortmund turns great. Nothing they can do about this instead of keeping this up, until one day players will begin to stay there.
In hindsight if all these players or at least most of them had just stayed, Dortmund would be a main contender for the CL title, arguably with a squad at Bayern/City level.