Marciano didn't have a style that lent itself well to old age. If he'd come back, he might've won, but then everyone would have been saying that he was afraid of Sonny Liston. It was a no-win situation. If he came back to fight Ingo, he would have also had to fight Liston, and no way he wins that in 1960.
No version of Marciano could have beaten Liston, not the 1952 version or any other. Liston was far too powerful and athletically superior to the Rock. Liston by ko inside of 4 rounds, more likely a tko but I wouldn't rule out a complete ko.
I like the Rock and enjoy watching tapes of his fights, but Liston would have been a bridge too far. He did the right thing for his legacy by retiring. If Tyson had retired in 1989, he would have had a similar career to the Rock but of course he carried on fighting. If the Rock had carried on and fought the likes of Liston, Williams (big cat), etc, then no way he retires undefeated.
That being said, I think Marciano would have stopped Johansson fairly easy. I also think the Rock would have been made to look a fool for a while against Patterson and maybe even suffer a knockdown, but I believe he would have worn down Patterson and knocked him out.
People need to accept in relation to Marciano, that your record isn't just an indicator of how good you are, but also how good your opponents were. Does anyone who thinks the Rock is the greatest, because of his record, really believe that he could have fought the guys that Ali did in the 60s and 70s and remain undefeated? You'd have to be high on crack to believe that.
A few predictions if I had to bet
Foreman ko2 Marciano
Liston ko4 Marciano
Ali w15 Marciano (almost a shut out)
Tyson ko2 Marciano (the Rock deserves to be ranked higher, but no way could he survive 2 rounds in a head to head with Tyson circa 1988)
JJ would have absolutely toyed with Marciano. Points win.
Dempsey v MArciano is a toss up, but I edge with the Rock
Louis v Marciano (again, Louis deserves to be ranked higher, but I can just see the Rock getting to him very late and stopping him. Say between 12- 15 rounds, with Louis well ahead on the score cards)
Lewis ko2 Marciano
Of course the above is pure conjecture, but it's how I IMAGINE the styles of each fighter coming together. Some fights I can't even imagine how they would go, like Holyfield v Marciano. I can't even imagine how the fight would go. I think Marciano was very fortunate to be fighting post Louis (yes I know he koed an OLD Louis in 8) and pre Liston.