I just did this two weeks ago:
Nadal, then Fed, then Nole/Murray changed the complexity of the game. The rest of the tour had to follow and now everyone or almost everyone in the top10 is a great athlete. A lot are amazing athletes. Novak always had the talent but he couldn't keep up with the big2 and he was forced to adapt.
Another good example of this is a brazilian guy named João Souza. Tall guy, big serve and clay specialist. Was a fringe top200 guy with ups and downs his whole career. Last year he decided to follow Nole's diet. He also found his body was intolerant to lactose. He cut gluten and milk and its' sub-products from his diet. Focused on his physical fitness, had deep runs at challengers in a row, reached top70 and a month or so ago broke the Davis Cup record of longest match in he Davis Cup, almost reaching 7 hours of outdoor play in South American summer.
Tennis became a war of attrition! Being great at hitting shots alone wont take you past the top200 anymore. Unless you are a big server. You need the whole package nowadays and a byproduct of this is that YES after those guys retire we'll have a weak era. Almost a whole generation was lost because they were trained their whole lifes to do one thing and then entered the tour and it was totally different. Some will reach their peaks very late, others will just stop.
Anyways, tennis is not a game of youngsters anymore and awaited this for more than a decade.