After the age of 28, very few multiple Slam winners in the Open Era have won more than one slam. The exceptions are Laver, Rosewall, Newcombe, Connors, Sampras, Agassi, Wawrinka and Federer. However, not included on this list are all-time greats like McEnroe, Lendl, Becker, Edberg, and Borg (because he quit early). Djokovic turned 29 just before he won the 2016 French Open.
In every sport, there are styles of play or particular positions which allow elite athletes to stay at the top. Bigs like Duncan and Kareem played significant parts in championship teams as they approached 40. Great hitters can be the DH in the American League and play just as long.
But tennis is different. There is too much running combined with amazing reflexes and hand eye coordination. There is no outside coaching or time outs. And no one to take your place on the court.
There’s a reason Michael Chang, Mats Wilander and Lleyton Hewitt couldn’t win Slams after their early twenties — their games were totally dependent on movement and consistency. As these physical gifts begin to decline, there is nothing else in their games that can keep them up at the top.
The Aussies played their Slams on fast grass. Sampras and Federer play with an effortless style that features a dominating serve. Agassi and Connors had the best returns of all-time. And Wawrinka dominates points with his unrivaled power and occasionally can get on a hot streak.
If Djokovic can’t regain the edge he had with his serve return (his biggest problem the past year), the small decline in his speed and stamina at age 30 will cause him much greater problems than other all-time greats, and prevent him from winning multiple future Slams.
Having said that, of course, he will immediately win Wimbledon.