Who's this mythical Prime Cormier?
Cormier had dogshit showings at HW against Monson, Staring, Mir, Nelson, and Stipe 2-3. Who even knows about his earlier career before Monson, but of his known career like half of his fights he looked beatable. People only focus on the times he looked good and pretend that's how he always looked. And let's not even bring up flukes that helped those like Barnett breaking his hand, Lewis coming off a war against a striker and on 3 weeks notice, or Stipe getting eyepoked on the side he got KO'd on.
The best thing he ever did was move down to LHW and start a fake feud with Jones as it allowed his first losses to come from a GOAT, while he was still "youngish", and forced him to (mildly) stop being a fat fuck. The writing was on the wall that a loss was coming sooner or later at HW the way he was looking. He was never some unbeatable HW, he got some very good timing and matchups that let him climb the ranks but it's no coincidence he left once he hit the top 5. And yes it's because Cain was champ, but not because they were friends but rather because Cain would've ran through him like DC openly admitted. JDS also had a very good chance of KOing him, DC didn't have the size or length to do what Cain did (not saying he may not have, just it would've been harder). Etc.
LHW Cormier > HW Cormier and it's likely not even close. And that guy still lost twice to Jones, nearly got KO'd by Rumble, nearly got KO'd/lost to Gus, and gassed/got hurt against a 3 day notice, 198lb, post-surgery Anderson.
Cormier was never unbeatable, in either division.