I would agree with your normally, but this isn't a normal personality. He is ultra-competitive, and given the bright start he's had in this competition, he has a genuine chance to break/set some records in the competition that he'll lose out on if he joins a club that's not guaranteed to have CL. The most important thing at his age is to not take any steps backwards and keep competing at a high level. He can accomplish that with a club like Dortmund and he'll still have plenty of time to make the next step up, albeit most likely with a much steeper price. Who knows what he'll end up doing, but I feel it'll be hard for him to let go of the CL, even for just a season and his age.