Hall of Fame
You didn't add this 2019 year to their age. If your pick is Djokodal retiring at the end of 2024/2025, then Nadal/Djokovic will be ~38.5yrs by end of 2024/2025.Well, not 39 yrs old. Nadal is 32.5 right now and Djokovic 31.5, so I believe they will retire about 37.5. Djoko said he want to play till he is 40, and Rafa... you just described it all "had many people thinking that he won't last past 25-26yrs old. Sure, they were badly wrong".
If Federer could last till 37.5, then Djokodal can too.
Your quote that "If Federer could last till 37.5, then Djokodal can too." assumes that they play the same style while being equally efficient. They don't (surprise! ). Though anything is 'possible'. But then again, playing until 60yrs old is 'possible' too.
But possible =/= probable.