No significant difference in surface does not equate easier to dominate slams. There will be more good players to adopt their games to be better on the surface. Sampras had a hard time winning on clay because his game, which was perfect for Wimbledon, has many holes for RG. He tried to adjust his games to better on clay but it affected him on glass. Vice versa for clay specialists who did not even bother to show up in Wimbledon. You have a set of players who were great on Wimbledon, another set on Roland Garros. Even for the faster hard court for USO and the slower for AO, there are more overlap of players but still, some stand out more.
Now on the medium pace hard courts, all the top 4 players are actually very good on it. Even the rest of the top 10 can play very well. Thus, it's not that easy to dominate.
How many players in modern era managed to win all slams? And how many managed to win all slams more than once?
I don't believe this at all. If every surface is the same then all it means is that the top players will all succeed in every grand slam, which has been the case since surface homogenisation came into effect from the ATP. There are far less upsets today in early rounds of slams because of this. Changing from 16 seeds to 32 seeds is also relevant here.
Let's look at the Sampras example. He dominated Wimbledon because his natural serve volley game was extremely beneficial to the grass courts of the 90's. Grass was significantly faster during this time, and as a result, enhanced this type of game style for those with the skill and variety to execute it efficiently. Think about Sampras playing on today's grass courts at Wimbledon and how racquet technology has evolved since the time he retired. Serve volley simply doesn't work and this is why that particular game style has pretty much vanished from the modern era and baseline grinding has taken over. It all comes back to the fact that people can't and shouldn't compare eras and players from separate ones. Wayyyyy to many variables to take into account accurately.
Nadal and to a lesser extent Ferrer, are also some more prime examples. During the days of surface variation, Nadal would be extremely hard pressed to prevail at Wimbledon. As it stands he has won it twice and been runner up on 3 occasions. Ferrer also made the QF last year and tested Murray immensely. It all comes back to:
- No variety in surfaces = easier to dominate all 4 slams, thus increasing a players slam count significantly.
As for the last question, Federer and Nadal have done it and Djokovic will most likely do it as well. This is only since the beginning of surface homogenisation. If it stays like the present day for a large period of time then I am sure the number will increase dramatically.
I am not undermining Djokovic's achievements either. These are the conditions that have been built around his rise to the pinnacle of the sport and he has adapted to them fantastically. He is without doubt one of the best players of the modern era. I just don't see any logical or rational explanations to compare them to players from previous eras when there are so many variables to take into account.