Personally (and this is just my opinion who's watched along and keenly followed Fedor's career from his debut in Pride), I think when he fought in the states, maybe for the last couple of years, he was doing it more out of 'obligation' and to finish his contract, than his love of competing.
I'm not sure his mind was 100% committed to fighting and training.
A big part I think also was fighting inside a cage and fighting using the unified rules. I think his training did not 'evolve' to be 100% aligned with his environment
Yes, this might not have been obvious unless he fought really good guys, I think he could have fought monson, ishii and rizzo in the cage with his 'lesser' motivation and training and still won, but when you fight hungry season, elite fighters like Hendo, Bigfoot and Werdum, that isn't going to cut it.
He was bound to lose even losing half a step.
Do I think the sport 'passed him by'? No. I think at his peak, he would have been good enough to fight anyone who's ever fought in MMA and do well, if not win. I think he was most suited to the ring/Pride rules (he is absolutely the GOAT in that environment), but potential wise, I think if he had the fire, he could have done well enough too in the unified rules/cage.
So a lot of his downfall to me, came down to a combination of the timing of the move the a cage/unified environment, lesser motivation, slacker training, too agressive fighting approach (like 'I just want to get this over with and what happens happens) AND the fact he was facing top competition to exploit these facts.
I offer no excuses as a fedor fan, a loss is a loss regardless, just reasons as what I think this idea of 'the sport passing fedor by' is indeed a myth.
*Knees to the head of grounded fighters
*Kicks to the head if both fighters are grounded
*Re-evaluate US MMA scoring