He wanted to leave as he was fed up being played out of position. Seems fair enough to me, not unlike someone leaving a job because instead of managing accounts, they were being asked to do the filing all the time.
Money is not the driving force, I'm pretty sure of that. Of course, the top guys who have the choice of Chelsea @£100K pa or Barca @£200K pa might well go for the Spanish side, but then so would every other single person in the whole world.
I'd rather stay warming the bench at Chelsea where I might get a chance to break into the first team and prove how good I was and win the title, rather than drop down a league and play every day and win nothing of note ever. That's just common sense. I think you're also forgetting that for every Rooney or Sterling there's a James Chester or a Michael Kightly, who really aren't earning the mega bucks despite playing in the same league. A footballer's career is a very short one, even if they were purely after the money I don't blame them at all for that: They're one bad tackle away from never playing again, and then what do they do?