Kasparov can become world champion again but in classic chess where he has enough time to devise strategy and tactics. In blitz and speed chess he played better against the top players like Nakamura, Caruana etc but he just needed a little bit extra time. In classic chess kasparov can surely win and restore his throne.
I strongly doubt that Kasparov can regain the title at any time control. The competition is just too strong. Heck even if Kasparov was in his prime today, I don't think he would be able to be the World Champ with today's players. Still, he can probably be a top 20 or perhaps even top 10 player if he tries his best. It will be interesting to see him to return - even for a short while.
But a lot of it has to do with whether he can accept the idea of being the punching bag for the younger folks. What will be interesting to see is that he can still win some games.
Indeed, Kasparov being an old guy that he is, he can't be expected to be the same young fella that he was. However, if he does his best, shapes up both physically and mentally, show up and compete again, his endeavors will not go unnoticed. I am sure that he will get more people to pay attention to the game. We have seen this in St. Louis to some extent, it will be interesting to see more. And yes, perhaps he needs to play at slower time controls and maybe he will do better there.