Whatever would've happened, I don't think he'd have had a second career or even as high a rating in the ATG lists for HWs.
In my own mind, after watching Foreman - Young, I am of the opinion that Foreman DID beat Jimmy Young. But, if you like lots of grabbing, very little effective punching by Young, lots of ducking, and a ref who penalizes George for pushing Young away when Jimmy grabs incessantly, then I can see why you'd give rounds to Young. Sure he decked George, but late, and after Foreman had thrown lots of punches with big power at him, trying to make it a fight, and had gassed himself. Anyway, to answer the question, if Foreman had won, he'd beat anyone out there, except Ali and Holmes. Those two would have had his number any day of the week. Too much speed and condition and defense and stamina and guile, etc., Foreman's defense was so-so, and he couldn't get either of them out of there early, and he certainly cannot outbox either to a decision in a 15-rounder.
George Foreman probably would have beaten anyone with the possible exception of Muhammad Ali, and the exception of Larry Holmes.
Virtually impossible to say.... I do think however that the prospects of him retiring regardless are still very high... I don't necessarily think that it was the actual "loss" itself that forced him to leave the game, so much as it was how he was " feeling." Foreman took 15 months off after the zaire defeat, and asside from Ron Lyle, whom he struggled with, was taking on softer foes in general... His head was not in the game against Young, and the drastic decision to totally end his career and become a preacher, gave insite as to where his thoughts were... So a victory might have improved his record, but I'm not convinced that he would have continued beyond that point.
I don't think Holmes could have slipped 15 Rds of George's punches, and GF wins via KO and is HW champ for 15 years or more.