It certainly would have boosted his legacy significantly, but In all probability it would have been overshadowed by the perception that Ali was washed up. Remember Foreman was already something like a three to one favorite going into that fight and there were many who feared for Ali's health. Had he beaten ali and perhaps even ended Muhammad's career, the win might have been viewed in similar fashion to Johnson vs Jeffries or Marciano vs Louis, with Ali being a tad more valuable due to his recent wins over Norton and Frazier.
Still beating Frazier, Norton, and a "washed up" or "shot" Ali would have done wonders for his legacy, look how Norton is regarded based off his history with Ali, if it were not for that rivalry he'd be in the same league as Quarry, Bonavena, and Chuvalo. I suppose you can include the fight with Holmes for his legacy as well.
I think Charles was still very good against Marciano the first time. He was no more washed up than Ali was against Foreman. Ali was good against Foreman so I think beating Ali (who was also an outstanding contender) would have been as great a win as Rocky beating Charles. If Foreman had then retired unbeaten he would have eclipsed Marciano.
No Charles was still 32 for the first fight with Marciano because he was born in July 1921. ALI was also 32 years and ten months old when he challenged Foreman.
After beating Ali, Foreman most likely would have grown overconfident in his power, this would set him up for a potential banana skin like Young or a much publicized showdown with Holmes who would have put on the performance of a lifetime. I think beating Ali or not, Foreman simply wasn't going to rule for long
Foreman fought the dumbest fight i ever saw V Ali. "If it's not working, let's do it some more"... And what the hell were his corner telling him, i always wondered ?. He looked an amateur in there not just with his tactics but his punching style,whaling away with sidearm roundhouse blows. Ali toyed with him from round 4 onwards. Subsequent fights with Lyle and Young in their various ways showed that George had his limitations. His victories over Frazier and Norton built a hype that he couldn't live up too. His comeback was admirable but limited in terms of who he beat. Probably the last guy you would want to pick a fight in a bar with, but GOAT...? NO way. Over rated IMO.
This is why Ali's victory over Foreman was so magnificent. Before the fight why would Foreman have thought that his tactics WOULD N'T work,owing to what he'd already achieved ? Muhammad's improvisation was brilliant
If Foreman had beaten Ali, then we wouldn't have had the post Zaire headcase Foreman. I think Foreman would have beaten Young, and Norton a 2nd time and ran threw the division until being finally being dethroned by Holmes.
Yes Foreman can be called overrated but what you mention is the hype that he had after blowing out Frazier and Norton. With the fanfare he received at the time, plus beating Ali even if he was seen as not so formidable, could he have not received the hype to be called the GOAT, or something close to it? Close to it because I'm sure there would be some that would still believe Joe Louis is the GOAT.
Holmes would have gotten to him around 1979, he would have had a six year reign.As long as he avoids Quarry, and maybe Bugner.