You're welcome to make your own hypothetical assertions. They're no more valid than mine. I think Cain would outclass Stipe. I invite you to rewatch his fight with Werdum. He was easily the better man before he gassed - Werdum did his whole training camp in Mexico City while Cain showed up right before the fight like an idiot. Sea-level Cain would not have much difficulty with Werdum, imo. Again a hypothetical but altitude is no joke. The Ngannou fight was meaningless - Cain was so riddled with injuries by that point that he retired then and there. He was coming off a close to 3 year layoff due to injuries and was fighting once every other year before that. It's like saying Fedor would always lose to Big Foot except that actually has less asterisks.
I concede that these are "what-ifs." It is my opinion that what Cain did to Rothwell, Lesnar, Kongo, fading Nogueira, and prime Big Foot was indicative of a superior talent. His manner of victory counts for a lot in my book even if the names don't jump off the page at you.