Well,
as I've said before, I am convinced that the vast majority of drivers that would be all-time greats, make it into Formula One, regardless of pay-drivers or sponsorship. Of course everybody needs at least some form to get started, but looking at the humble backgrounds of f.i. Alonso and Vettel, I think it even more proofs that cream rises to the top.
Although I can believe that some potential top-but-not-all-time-great drivers have not made it because of finances (Robin Frijns, please don't become one of them!), I do still believe that looking into that category, they do represent some of the best drivers out there. Lower then that, and it becomes really tough to call, and although I'm a big fan, I'm sure someone better than Kobayashi is not in F1 because of sponsorships, but I believe in general, it wouldn't be that big of a difference. Also consider that the gap between the best and the 'worst' drivers of today, has (most likely) never been so small. The gap between the best 20 F1 drivers in the 90's was considerably bigger than today.
Do I think someone of the level of Vettel/Alonso/Hamilton is not in F1 today because of sponsorships? I do not think so.
So, can it be that some better drivers are not in F1 today? Sure.
But would those better drivers have been considerably much better? Barring 1 or 2 (statistically, not actual examples), I don't really think so.