There is a yin and yang to everything.
All the majors (except the French) were played on grass up until 1975 when the US Open went to clay (Har-Tru )for 3 years.
In the mid 70's, many US tournaments switched to clay - apparently to give the fans longer points, and maybe even to draw Borg to their event.
When the US Open left Forest Hills to move to the new USTA center in the Queens in 1978, the US Open went to fairly fast hard courts.
The last Australian Open was on grass in 1987. (The ASO "grass" was usually burnt dirt by the end of the tournament - players boycotted it because of the time of travel [and purses were much smaller then], time of year [used to be played pre-Christmas to about New Years Day.] It was played on a relatively "spongy" type of hard court surface known as Rebound Ace for 20 years, then replaced by Plexicushion Prestige which alsol has rubber and latex in its formulation. (I also remember players comlaining that the Rebound Ace would get "sticky" from the hot Australian sun.)
Even at Wimbledon they have changed the type of grass, mainly to improve the terrible wear that occurs during the fortnight. But the newer grasses are not only more durable (resulting in less bad bounces), but give a slower surface.
You asked if Sampras would not do as well on today's slower courts.
Well how about this - what if US and Australian Opens had stayed on grass, like they were when he was born?
What if the pros had insisted on staying with wood racquets, just like Major League Baseball has insisted on staying with wood bats?
How much have the balls changed since when they were manufactured in the US and UK?
How many titles for Rafa if the US Open had stayed on clay?
How did Borg not win a US Open when it was played on clay? (Har-Tru)? [Couldn't resist this.]