Longevity is yet a significant measure of greatness.
Rosewall dominated at majors for four years.
In Tilden's prime there was no French championship for foreigners. He did win the World's Hardcourt Championships on clay.
We should also mention that Tilden didn't travel to Europe in 1922 and 1923 for the WHCC or Wimbledon, and I can't say that I blame him considering all the travelling by boat he would have had to do every year, while he wasn't earning a dime for playing tennis (as he was an amateur). Tilden was clearly seen as the world's best player in the first half of the 1920s, and he expected his biggest challengers from other continents to go to him. The French Musketeers (Lacoste, Cochet, Borotra) certainly answered that call with a lot of success in 1926 and the years following. Tilden started getting his own back in 1930 and then as a professional from 1931 onwards. Tilden had Cochet's number in the professional game.