BVB replacing Hummels well
Our boy sure knows some loyalty, but in footballing terms he's a downgrade let's be honest.
If the news is right (not seen any reliable german sources reporting it yet), I just don't think it's the ideal decision. For one, BVB are Buyern's closest contender which means they can't afford individual slip-ups. If he doesn't impress and fast, he will most probably end up behind Soktratis, Bender, Ginter and Subotic (if he recovers from his perennial injuries). Not only would Bartra have to adapt to a new league, but also work on his game on an individual level. It's a slow gradual process. A move to a lesser Liga club would have rendered a more generous platform to work with, and without the pressure factor.
Unless BVB see him as a long term project, and invest the time and effort into developing him over time. But given his flaring weaknesses (limited pace , lack of strength), not sure if the ceiling he'd reach would be worth it from the club's perspective. Or may be he can become a solid squad player for them.
In any case, I wish him luck, and that he surprise us all.