Most pro boxers at the highest levels couldn't hope to knock out Mayweather, nevermind someone with zero professional bouts to their name.
Floyd accomplished a hell of a lot in his boxing career. People talk shit about him being a dingbat, as he isn't the most eloquent person, but his ring intelligence is unmatched in boxing history.
He knows where to be and when, in the ring. He takes a few rounds to size you up, figure out his opponents game plan, and then executes his own.
Every single time he was in a tight spot during a round, or at a pivotal point in a championship match, he delivered when he HAD to. Every time he needed a round to solidify a decision, he got it. Every time he was hurt he figured out a way to position himself to survive and go on to win, against the best opponents in the world.
So even if McGregor went out there and was 4 times better than what he showed Saturday, and actually was getting leverage on his punches and put Floyd in some real trouble, Mayweather had the capacity to deal with it and adjust accordingly. Conor could of thrown 750 punches, he could of bothered him with a stout jab, none of it would have really mattered because Floyd is good enough to respond to it, and dismantle his attack.
All he did was wait the fight out. Everyone including Conor knew he wasn't going to be able to compete in the later rounds. All Floyd had to do to adjust in that fight was simply bulldoze his head into Conors chest from the 4th round on, which completely nullified his offensive capability due to clinching and forcing a reset. This also quickened his fatigue accumulation.
Conor never had a chance in this fight, no fix was in besides fooling people to pay 100$ to watch it.