If not for the 2 losses he had with Leon Edwards for sure I would not doubt Kamaru Usman so much as Khamzat Chimaev has proven that he can destroy anyone in his division and if he can exchange with Gilbert Burn in a stand-up then for sure he can also exchange on the stand-up against Kamaru Usman in equal footing with him and for sure Khamzat Chimaev also have the better wrestling in my opinion, so there is no doubt for sure,
Firstly, Kamaru Usman is a great fighter. But as you have said before, maybe he is past his prime. They both also have a large age difference. Currently Usman is 36 years old and Khamzat is still 29 years old and the average fighter is considered to be in his prime when he is 30 years old. Age is not always a benchmark because there are still very great fighters over the age of 35, such as Texeira, Miocic or Jones. But not all fighters are blessed with a long prime, especially when they have experienced brutal defeats.
Second, Usman moved up to Middleweight (correct me if I'm wrong) which of course was a slight advantage for Chimaev because he didn't have to cut too much weight.
And if you look at Kamaru Usman's side, in my opinion, if a fight is decided suddenly like this, each fighter will not have the opportunity to study their opponent in more depth and prepare a training camp to train intensively so they have to rely on experience and their respective fighting skills. And Usman, as a fighter who has fought more than 20 times in the UFC, certainly has a slight advantage here.