Chopin buy the DVD above, watch and report back to me. It's all different from era to era. Laver and Rosewall play a different game but an amazing game. They volley so well, cover the court so well and hit such great angles. It is an interesting game to watch. Today players are very athletic and hit as hard as the human reflexes will allow. I don't think we can go anywhere as far as speed goes. Tennis has reached a peak in speed. Where do we go from here?
As for comparing players from other eras, it is like comparing apples and oranges. There are so many things to factor in. Grand Slams are not enough. I have a big collection of tennis books, some very old. The same discussions come up in every generation of books I have. Comparisons of players and playing styles. It is interesting that one clear pattern is the dominance of baseliners being followed by serve and volley dominance and then baseliners again and so on. Can that happen again?
Players used to skip some tournaments because of money or the amount of travel. Many players in the early days of tennis would not make the trip to Australia. Someone like Tilden would have accumulated even more slams. Also Davis Cup used to be more important than the big tournaments. You can also factor in the two world wars and of course pro vs. open era tennis.
It is clearly impossible to label anybody the GOAT.