No single player is bigger than the club he plays for....but i don't think Rooney leaving is a good thing for the club. When you keep losing your top players it makes it very hard to convince the likes of Sneijder, Messi, Villa etc to sign for you. You don't want to lose your top players just when they are at the peak of their careers.
Wenger sold our top players (Henry, Pires, Viera, Overmars, Wiltord, Anelka, Reyes etc) just because he thought the players we had then had potential to win more trophies.