Joss has a point too, however: would you say that Hleb is as good a player as he was at Arsenal? Was he ever that good again? I'd say no, not by a long shot, and he's nearly the definition of the words "washed up" now.
Noone denies that, but even if the players are 'washed up' now, many players left Arsenal and succeded, maybe not individually (And there's no guarantee they would've kept their level of performances had they stayed, you can never guarantee that), but it's a team sport and they've won trophies with their teams.