If somebody had tried to intervene in it, they likely would have been shot and killed, in addition to Floyd still being killed.
The broader media narrative would have been that the police had apprehended a man who was on drugs and/or had been involved in a home robbery when his friends tried to stop them.
The police would have been hailed as heroes and what is basically the right-wing narrative would have become the dominant one instead.
There was a very similar police brutality case against a Rom man in Europe a few years ago (I think it may have been Germany specifically, but there's a few similar cases). This one specifically was almost 1:1 with the method of brutal restraint. The victim's friend or family member, I don't recall, tried to intervene and got beat for it. He didn't die, but the one who was kneeled on did. So the "likely" is 99.9% true.