FWIW I’m a big boxing and MMA fan (more boxing), and I’ll point out I’ve seen UFC fights stopped in similar circumstances. Hell–Rousey-Nunes was stopped with Rousey having never left her feet. Rousey briefly tried to contest it, but any fan watching knew it was a good stoppage. In boxing, Andre Ward and Sergey Kovalev had a bout earlier this year–for three light heavyweight unified titles, that was stopped when Kovalev got staggered and basically was “done” on his feet, he had never been knocked down. No one seriously contested that stoppage and it was very similar to the McGregor stoppage.
There’s been lots of boxing bouts stopped in which the loser never left their feet. People die in boxing rings. My understanding is only 5 MMA deaths have been recorded in “sanctioned” MMA fights, none in the UFC (all were in minor promotions.)
There’s a lot of data on boxing deaths (link), and while boxing was far worse in the past it’s still a deadly, deadly sport even in the 2000s and 2010s.
Part of what made boxing so deadly in the past was the desire by fans and the refs to “let a fighter finish”, so you’d see guys not only knocked down once, but 4-5 times in a round and allowed to keep going. Tremendous, tremendous damage can be done to a person in a situation like that. And the reality is if you’re being manhandled that way and you, for whatever reason, can still stagger to your feet, it’s almost unheard of you go on to win the bout. So there’s a lot of good reasons to just stop fights like that.
The reality is the ref has the power, and the responsibility, to stop a fight when he determines one of the fighters can’t properly defend himself. The reason for not ending a fight anytime someone gets knocked down is that some knock downs are really just that–the guy is knocked down but still has legitimate fight left in him. But it can also be true that a guy is still on his feet, having never been knocked down, but have no fight left.
If Connor had the ability and wanted to stay in that fight he needed to have kept counter punching, needed to have shown some ability to dodge/defend punches, needed to not start basically blocking with his face, should’ve tried to wrap Mayweather up etc. The fact he did none of those things is strong evidence he was in trouble, I’ve watched tons of boxing and you could see his legs were gone, you could see his hands too far down etc. He was in in trouble and wasn’t effectively defending himself–textbook scenario for a stoppage w/someone still standing.