I don’t Marciano beating Ali and nor do I see him lasting any longer against Foreman than Frazier did.
Frazier's left hook was his signature, but not his only good punch. The famed left hook clouds the fact that Frazier's right hand was an integral part of his style - it had to be, to form what is likely the most elite examples of swarming mastery to be seen still today. His style and demonstrable success emerged from his ability to seamlessly integrate a bob-and-weave defense with a relentless pressure offensive. He was agile, rhythmic, and ambidextrous in his timing and versatile punching. Together, they produced a fighter greater than any single trait could suggest. I view Frazier as an upgrade from Marciano. I prefer his more fluid style and tighter consolidation of skills to form a more polished and complete package. His cleaner entries into the kill zone and means of becoming innately in tune with his opposition also stand out as underpinning superior ability, in my opinion. I'm a big fan of Marciano, and I would not, like some might, assert there was a massive gap between him and Frazier. Your post does a sound job of evoking the kind of thinking that needs to go into such a comparison. It is actually quite close - not a slam dunk on either side, I'd say. The open questions I have about Marciano involve whether or not his power and toughness would hold up in a superior pool of prime talent. Frazier outshines him in this respect, with a rack of wins against prime opposition that likely more than doubles that of Marciano's. In a similar vein, we can wonder how effective Marciano's brand of pressure-fighting would be against Ali in '71. In Frazier against Ali, we see how Joe's upper-body mobility and agility played a huge part in taking the win. How does Marciano fare comparatively? Of course, these questions do not yield definitive answers. However, I do think they introduce significant factors when assessing the levels and superiority of boxers.