The protests from the elite players regarding the one companion restriction is probably one of the best indicators of how those at the top have stayed there so long. With tennis income so insanely top loaded, the big 3 on the men's side and a few women are taking in such a large percentage of the prize pool + endorsements it's kind of like the Power 5 conferences in football (or even Bama/Clemson/etc competing against Wake). Resources have become unlimited for Roger/Novak/Rafa/Serena, so even if their competitors are gaining on them in fitness/raw talent, the access to the latest in training, nutrition, recovery, medicine, travel etc help slow the fundamental decline you'd expect as the new generation should begin a takeover.
Golf obviously has a few guys earning a disproportionate amount of overall income too, but most of the silly money is endorsement based and the top 100 on tour last year all cleared $1 million in prize money. You hear a lot about the younger guys (Ricky/Justin/Jordan - guess they're not so young anymore) sharing houses at some events and haven't heard a single complaint about "bubble" restrictions in the PGA re-launch, so unless golfers are more private about their concerns it would suggest the support team in tennis is much more significant than golf.
Or maybe this explains why Tiger hasn't returned yet....