Then I'd be fine with the likes of Edberg, Stich or Mac still going deep on clay.
Oh for sure, I think we can safely say Roger would have never won the career GS in the 90's and Nadal wouldn't have by this point in his career either, he still could if he continues to adapt his game, but I don't think he would have won it either.
Although you could make the argument that Agassi winning on all four surfaces, yet not even being the best player of his generation meant that it was a weak era. I don't believe that, but I bet that could be very well supported.
He proved his versatility in a specialists era, while Fedal have proved the best at dealing with the homogenized conditions. All three had a bit of luck anyway.