Predictions about Djokovic’s future success do not make him the best tennis player. Only time will tell if he will be. Saying he’s the best tennis player of all time now is premature and thereby incorrect.
The best tennis player should be determined by his/her number of wins. That’s how it is all throughout the sporting world; Djokovic is no exception. The goal of a sports game is to win. If your talent and skill don’t translate into wins, you’re not the best, because you’re not achieving the objective.
Finally, Djokovic is arguably (possibly) the best male tennis player. Sports often choose male and female greats, but there can only be one all-time-best-tennis player, and there is formidable female competition such as the Williams’ sisters.