Yes, he was already on the decline before the Usman fight. He was probably on the decline by the WB rematch (definitely by the Maia fight).
The best fighters of all-time to me (Anderson, GSP, Bones, Fedor, Khabib) are constantly building wide range of skills that work against variety of opponents, this way when their athleticism starts to falter (not as fast/explosive/powerful) they can use technique or adjust strategy based on the tactics needed. How long you can stay elite is a question of your ability to adjust your strategy/tactics to mesh with your declining athleticism. Eventually everyone will lose enough athleticism or skills where they are no longer elite, but you can lengthen your time as an elite fighter if you build a game that will be able to adjust as you get slower and lose cardio. Otherwise you become so predictable that guys would've killed years ago have a blue-print to pick you apart.