Hypothetically, if he was super fast and untouchable like Ali with more facets to his game. But he's not. He's bigger than everyone he fights and his division isn't talented enough.
Also I wouldn't really rate Usyk win that highly for Joshua personally. For me this fight is kinda loss-loss situation for him as far as credit goes, even though I'd like to see it. If Joshua wins, he was supposed to - nothing too impressive about beating a much smaller guy coming up from CW. Unless Usyk goes on to do some crazy things in the HW division such as beating Wilder, that's kinda where I stand on it at the moment. And before people try to argue over this point, how many people are legitimately thinking that Usyk will win? Not many is the answer. Majority of the people think he'd lose, and badly. The few who are giving Usyk a chance are pretty much Usyk fanboys and EEs.
It is about winning. If he keeps on winning and beats Wilder or Fury and has some defenses he could be number one.