I think Justin is nearing the end and the difference in mileage is going to show up here. Like, if you make the analogy to a car, Justin has been taken off-roading through harsh terrain the majority of his career. He's taken insane amounts of damage, even in wins. Fisiev is a talented striker, and just WAY less shopworn. He didn't "lose" to Green either. He won a very close decision that could have gone either way. And Green is a hell of a boxer, or at least was a couple years ago when that fight took place.
It's possible it goes that way with Usman, but literally ONE head kick is what we're talking about. If that kick doesn't land with 1 minute to go in a lopsided fight up until that point, there would be ZERO talk of Usman being "almost done". He threw Leon around for most of 5 rounds. His cardio was on point, as was his wrestling. I guess we can say Leon will "fight better, with more confidence"...but I'm not sure how that catches him up in the gap in grappling ability. I think it's likely this is similar to a Nunes/Pena situation where afterwards everyone is saying "Ahhh why didn't I take advantage of that line??"