This is not good for tennis. It creates the perception that none of the younger players are worth watching. Also, it is boring to watch Djokovich win. A million ground strokes and he makes fewer unforced errors than his opponent. Wake me up when it is over. Federer and Nadal are no different. There is no one in tennis today who creates the excitement of Pete Sampras or John McEnroe.
Just as the 100mph fastball in baseball has made it boring (to some), the tremendous groundstrokes make serve & volley unworkable in today's game. It's a pity. It's time to change something...I don't know what.
As for the G.O.A.T., that is an un-answerable question. Longevity is a part of it, but one has to look at dominance over a shorter period against outstanding competition, which brings Laver, Sampras, Becker and a few others into the conversation.