Who besides Medvedev (and maybe Berretini)? Btw, I'm not sold at all on Medvedev given the way he lost to Nadal - with Nadal basically flailing and putting the match on a silver platter for him early in the 5th - and also given this strategy of playing 30 feet behind the baseline. I think he really missed a chance to stake his place by losing that US Open final. I'm telling you in the long run he will deeply regret it as a missed opportunity.
I'm sure he already does, and he should. He had an opportunity because he was in the final. He also had a very tight Nadal not playing his best tennis.
Not necessarily also-rans, just not players on Djokovic's level.
There is a solid chance that we don't see anyone on Djokovic's level, or Federer's or Nadal's, for some years to come. However, the Big 3 are all in their 30s. Age slows them down, and even Djokovic is showing a little more inclination to injury. They lose a step, and the youngsters have less fear of them. Of the Lost Generation, the real talents were Dimitrov (underachiever) and Nishikori (oft-injured.) I think there are a lot more players with promise in the 18-24 or so range. Some will disappoint, never realizing early potential, some will be injured too often, but I think that there are enough of them for some cream to rise to the top. Felix is 19, Shapo 20 and Tsitsipas 21...they are far from peak years, in tennis terms. Zverev could yet realize more potential. This is not "wishful thinking" on my part, in terms of blocking Novak...though it IS a hope that there are real tennis stars in the near future, as a tennis fan. As I mentioned above, and you agreed with, it doesn't take a ONE to cause trouble, if there are a fair few potholes in the road. Look at the past 12 months for Djokovic, since we're talking about him: Paris-Bercy - he lost in the final to Khachanov; YEC - he lost in the final to Zverev; RG - missed his chance at the final to Thiem. That's not one player stopping him, but the field of younger players. I actually think that it's you who is trading in the "what is now will be for the next 5 years." Certainly, no one has gotten rich underestimating Novak Djokovic, but to think that there will be no challengers for the next 5 seems short-sighted, to me. Same goes for Federer and Nadal. As to Wawrinka, I think he's mostly done, and I don't see him taking out Novak in any more Slams, nor winning any more.