I feel Usyk is one of the best boxers in the history of the sport. But I don't buy that he held off knocking them out. He's always fought the same way: conservatively at the start, getting the feel for the opponent and his game, and turning up the pressure as the fight goes on. He's a very smart fighter, doesn't take unessecary risks. COULD he maybe have KOd Joshua? Yeah, maybe, but then he'd have had to have left himself open and risked getting caught with something big himself. Usyk doesn't fight like that.
I think Usyk will retire and be finally be given credit for great chin. He is a very good boxer with very good footwork, excellent stamina, with a reasonably good defense. Maybe he doesn't get enough credit for the stamina, and the chin. Never down as a pro or amateur that I am aware of.