Burnout is real in tennis, and usually injuries or players are just plain sick of the grind that makes early retirement easy for the elite players long before their game has really faded away in many cases. I think the majority of All Time greats especially before the Big 3 if they were relatively healthy and motivated could have stuck around longer and still been relevant dark horse contenders. I guess the real issue with most of them is if they could stomach being 25th+ in the world instead of Top 5 when they do slip a notch.
I think the Big 3 motivated each other as much as anything for that All Time Slam record in these later years. They still don’t seem to be unmotivated or sick of it and I think that’s the ingredient that hasn’t existed in the past. For Federer and increasingly Nadal it seems simply their bodies are letting them down. That along with the fact they are so rich a guy like Roger could fly his family with him anywhere in the world in first class accommodations, private jets etc whereas 30 years ago that kind of wealth might not have been as attainable or feasible for a family to travel like his did even for the elite players.
In the past, a player like McEnroe said he played Doubles specifically so he didn’t have to practice. I’m not sure he liked the grind as much at the time. Then you got the practice freaks like Borg that burned themselves out practicing 6 hours a day. I watched a documentary about Borg and McEnroe’s rivalry. They talked about how Borg had to have everything perfect with his on-court work and routine before he felt like he could play. Hours at a time. I’d imagine some of these obsessive guys put in more hours practicing than if they played even and it takes a toll. Borg’s early retirement with 11 slams at 26 when he was just 1 slam away at the time from the record from record tells you everything you need you to know about burnout.