I could say the same thing to Fred, unless he is actually that old to have seen him play.
I'm not one for taking everything ex-players and commentators are gospel, and I often think the line "So you think you know better than [player X]" is reminiscent of the Flat Earth Society trying to crush debate by telling people to look around and prove it isn't flat, but in Grace's case I think we can make an exception, since information is so hard to come by.
Along with that, when you do apply science to Grace's career like PEWS has done, in the context of his peers, Grace is the best player ever. And we should only judge players by the context of their era because as someone who can express ideas far more eloquently than I once said, Sehwag doesn't bat to impress you enough to think he could take on Dennis Lillee, he bats in the style he finds most effective to score runs and win test matches for his side.
It is completely irrelevant what Tendulkar and Grace would do in each others eras, because they will never have to play in those eras.
Lastly, your "talent" argument is a classic case of using something which can never be quantified to justify your argument. You cannot prove Tendulkar is a more talented player than Grace, and even if you could it would still remain entirely irrelevant because we are measuring success, not potential.
What you basically said was you think Tendulkar can piss higher than Grace and that he has a massive penis, despite the fact you would be arrested for attempting to prove this is the case.