It is also amazing that there is a single 'big' club who does repeatedly the same.
BVB could have given Hummels three options: 1) sign; 2) leave immediately for a foreign team; 3) be dropped.
Otherwise, this was going to happen. Bayern is a significantly bigger club than Dortmund, who wins far more trophies, and gives bigger wages to their players. Obviously, Hummels would leave BVB for Bayern, same as Reus* will do in a year or two.
* I know that he didn't want to go to Bayern back then, but that was because he wasn't going to start for Bayern. If one of Costa/Coman doesn't play good enough to be their starter for the next following years, Reus can easily change clubs.