Really interesting points and nice articles by the way, but you are forgetting something here, is the environment that is forcing the players to a more phisycal game. They are better athletes not because they want to, but because they need to in order to success in the current tennis world.
Racquet technology among other things, has made points longer and longer, (surfaces slowing down is also a factor), i know that a rally between Vilas and Borg on clay could last ages
but with modern racquets and the immense amount of spin they generate the retriving skills of players are increasing exponentially. You can`t come to net anymore unless your approach shot is almost perfect or you will get passed like a club player. Grinding is the new style, and the better athlete will come out on top (provided he has a good skill set). The greatest proof IMO of the physicality of the modern tennis is the Us Open final last year. That third set was a massacre of epic proportions never seen on a tennis court before and i am very doubtful that the average player of the 50`s or 60`s would have handled it better than modern players. In this year Us Open Del Po blasted some balls that it is hard to believe that someone could even touch them, but Djokovic returned them with interest, it was beyond crazy and i think if you watched the match you must agree on this, yes the racquet is a big factor but the physical condition has to be excellent to say the least. For example Federer had to increase his upper body strength because Nadal`s topspin was killing him, i have heard that he needed ice packs on his right shoulder and chest after 2006 rome final.
However, i do believe that the great players do what it is necessary to do in order to win, so the likes of Laver, Rosewall, Hoad, etc would adapt their bodys and games to be succesfull in the modern era. But they would be totally diferent players, that is for sure, probably with a two handed backhand, a much more baseline orientated game, they for sure would volley much worse, etc. Would they be as succesfull as they were?? i honestly don`t know.