Well, You could argue that Spider was losing his edge a few fights before he lost.
Similar to Fedor. He was still winning fights but any fan can objectively look at his fights with Mark Hunt..or even Lindland, HMC, and Rogers and say “he isn’t the same anymore” but he’s still getting the job done so it’s not really something you harp on.
Anderson at some point didn’t have the same edge as the guy who destroyed Franklin, Hendo, Griffin. He still won some fights and then he lost and not just his edge but his mystique is removed. After Shael, and then definetly after Weidman everybody in the division thought they could take him.
It’s an edge but we also don’t know what changes are made in training. If they even train as much. An injury took something away..or simply reflexes, timing, and calculation just aren’t the same. There’s a mental aspect.
I think in Fedors case, his body didn’t break down because of of age but his style deteriorated because he changed the way he trained. Now his age is a factor as well.
Anderson Silva might’ve just got old.
As for CroCop. I just don’t think cage fighting was his bag. He’s out of prime because he’s old(still with some good recent wins) I don’t think he loses to most of those guys in a ring. Immediately different fighter and being real..the competition wasn’t better in the UFC. Atleast not at the start. He was fighting lesser fight but he was a completely different fighter even vs the cans in UFC as well.