They are. But take Messi out of Barca and they'd struggle, too.
That's the Spanish NT and we've done allright
But I think they rely on him more. Take Cronaldo, Falcao and Messi out of their teams and Barça would still be La Liga's top goalscoring side.
It's also a leadership thing; Cronaldo is the only one to give a damn nowadays, as witnessed yesterday. That was the most apathetic Real Madrid game I have seen since Mou's been managing them.
I don't think it can be measured over one odd game here and there. AM would struggle, yes. But for Barca and Real, I don't think they would struggle over the long term if CR and Messi got long term injuries let's say. Barca would still be superior, but Real wouldn't be that far behind.