It's still tough to transfer to your clubs main rival. Dortmund and their fans deserve better than that
It is in a way yeah, but Bayern pretty much allways been the bigger club, and the main natural destination for German top players to go to. Just been last couple years where Dorthmund been more competetive and made enough stake as a club to halt that automatic desire for Bayern german players used to have (used to be routine some Dortmund/bundesliga players wait out their contract just to go to Bayerns bench), Reus choosing them over Bayern was a big win for them.
But alas, they kinda settled their dominating presence a bit more firmly this season, and also consistently shown they can go to CL semi final&final. Its a little bitter to lose him to Bayern for sure, but they have these clauses on their contracts for a reason, they cant afford to have biggiest salaries around like Bayern, and set these clauses to appease their players ambition, like in this case.
P.S Dortmunds main rival are Schalke, even if they are better then them yeah.