I kind of see what you're saying, but I think there's as much - if not more - of a danger of "native talent being wasted via transfers to teams that don't play them" in the Premier League itself without worrying about the odd player spending a few years in Spain.
Did it really help Scott Parker, Shaun Wright Phillips, Scott Sinclair or Adam Johnson to go to Chelsea/Man City?
D'oh, not being clear today. I was trying to say that it doesn't matter whether the team is at home or abroad, if you are playing somewhere you love, getting games every week, and have no real reason to move other than for a(n even) bigger wage, just because the big money offer is there doesn't mean you should take it.
All the examples you've said are fantastic, Adam Johnson and Scott Sinclair probably the 'worst' culprits if you see what I mean. Get stars in their eyes and ignore the fact they're going to be sat on the bench or worse week after week.