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).
Many fighting styles are built around athletic gifts; if you fine-tune those skills specifically around those traits then you become piegeon-holed into a certain style and approach.
Woodley's downfall was that has always been a meat & potatoes wrestle-boxer that occasionally blitzed with single power strikes (overhand right, left hook)- as he got older and began to lose speed/cardio his tactical adjustment was to put himself on the cage and minimize his own footwork/movement to conserve the explosion. His thinking must've been "I can't risk getting tired because then I'll get picked apart - I need to make my opponents fear my double-leg/right-hand and if I feint enough and am fast enough they don't have a choice but to respect it."
This worked well as champion for a short time against specialist opponents but we definitely saw the cracks against WB and Maia - he was so low out-put that he was giving away rounds, so if he didn't get the finish it was ugly/slow fight. Additionally, as he lost speed and ability to make reads intuitively he would start being a split-second behind and the kill-shot that used to land clean would miss and he'd be stuck in the clinch. I think his messed him up baddddd mentally - when you are so used to hitting your opponent and you keep missing it starts making you second guess yourself. Everyone looks great on pads, it's how you look against a dangerous opponent that will hit you back - Woodley looked great in training videos but against dangerous opponent he got shook. I read interview that he got kicked in the calf and it blew up his leg like a watermelon; I'm sure that + getting dropped, cut, mounted 2 minutes into the fight completely shook him and made him go into pure-defense mode, looking to survive and land a hail-mary strike (very low chance). Credit to Burns for fucking him up so quickly but the Woodley that beat Lawler would've never let himself get walked down and chewed up so easily; he would've been loaded with lighting trigger counters on the right hand every time a naked leg kick got thrown.
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.
