it is possible that some day somebody will come and get 15-17 slams but nobody is ever going to be able to win 7-8 RG crowns.
lot of those slams will have to come on quicker surfaces. wimbledon is the least demanding of all slams so if you have a monster serve, you need to rack up 6-7 there.
and then you need to rack up another 8-9 on hard courts. again that big serve will come in mighty handy. and next thing you know, you are in the ball park.
you just cant get away with that on clay. red clay negates your serve and you are left to fight for every point.
the surface and the game is just too demanding on the red clay. that is why some of the biggest names ever could not even manage a single title at RG.