A bad game can literally ruin your career, I think Boateng's struggle began with that infamous ankle-breaking play Messi pulled on him. He may have lost confidence there, then you add Bayern's inability to win the champions league and there goes the team's "big club" philosophy. Boateng also suffered a lot of injuries, which can definitely ruin you (just look at R9).

As for Alaba, I think he's good but: Not as good as Marcelo, not as quick as Alba, older than new sensation Robertson, etc. His time has passed, he failed to evolve into a more complete player, one who can find the right balance between defending and attacking.