This is silly. Because
El Mariachi is his best film yet, no film he makes can be better than it? It's like saying Pulp Fiction is QT's best and he'll never be able to equal it. It may, in fact, be true, but until he's dead saying so is COMPLETELY BASESLESS. What are you talking about?
You could say RR's had plenty of chances to equal
El M. and messed it up, and that's sort of understandable. Still though, you wouls be talking out of no where, as there is awesome buzz around this movie LIKE YOU WOULDN'T BELIEVE. There's more than enough chance this film will be RR's best. Heck, there's a chance if done right this'll be the best film ever! And, judging by the word on the work already done, there's every chance it will be done right.
In conclusion: What? That's silly, and drop that. We're not talking about a film that any of us have seen, much less a film that's been made yet. We're talking hypothetically, and, in those terms, you have no valid arguments against it.
Worry not, dear friends. All is cool again. 8)