Nadal is a great champion, so is Djokovic, and Federer.
The reason or reasons Bjorg or Henin stopped their career are some type of lack of motivation.
The reasons tennis players stop playing at their top level in their thirties is not a lack of stamina ,but injuries.
A lot of progress has been made in the prevention and rehalb process
And Nadal has had his shares of injuries where he may have found andw well of motivation
Like Federer and Djokovic he has a very strong and healthy support system.
This may not have been the case of Henin and Bjorg at the times.
Connorz is another example of a champion going deep into his thirties.
Navratilova went on playing doubles in her 40 's and winning
Number 17 in the world today in women6 double ( and former n 1)
Kvita Peshke is 45.