I'm not denying anything, people can compare what they want, but if you want it to look the least rational, you need to compare things in the same setting, and football nowadays is not the same it was in the 80's nor like it was in the 60's and the same goes for F1. No one in their right mind would try to objectively explain us that Einstein is smarter than Aristotle or Isaac Newton, people can have their preferences, but to try and be objective about these issues is pointless. In the case of F1 the Fangio lovers will say that the cars were impossible to handle and that Schumacher would be fecked, the Senna ones will say that Senna raced against some of the best pilots ever and Schumacher never had reasonable opposition, etc...
And for the sake of it, I'm more with the crowd of the "let's wait and see" because Messi's career is still in progress. If he does a Ronaldinho no one will put him alongside the greatest ever, that's for sure, no matter how many goals and assists he does in one or two seasons. If he does continue to grace us with his skill and win titles for his teams, then with every reason he'll be up there. He doesn't have to emulate Maradona's feats, it's possible that not even Maradona could do it in today's conditions. He just needs to remain some notches above every else (like he's done for the past two seasons) for sometime more.
Even then, someone will be arguing that he's better than Pele, but second to Maradona, while some other fool will show out of the blue telling us the the order is Messi, Pele, Maradona, or whatever people want to construct in their heads.