I strongly disagree.
Not everyone responds to damage the same way. One fighter might be okay going through wars over and over again and others might be toast after one tough beating.
I also kind of think you're missing the argument of the initial claim; it's not that he magically left his prime in the sense that he lost his attributes, it's more of a psychological thing.
The beating was so bad on him 2xs in a row that it mentally fucked him up.
I don't have the stats on hand to back this up, but purely from the eye test and his up and down record JDS went from being a super confident striker to being much more tentative.
That's one of the tell-tale signs of a fighter being past it, having an unwillingness to throw.
He did have some good wins after Cain , like Mark Hunt and Derrick Lewis, but he was very clearly more aggressive and accurate pre-Cain.
Side Tangent: Cain is the most overrated fighter in the UFCs history. His resumé is nothing spectacular at all and it's harious to me when people include him in HW GOAT conversations.