Just watch that video - any half-arsed boxer would have seen that coming and moved.
I don't think it's as easy for a boxer to do that in an mma fight as you think it is. Neither do most pro-fighters who comment on such things.
I'd love to see Mayweather have an MMA fight with McGregor. The money would be there, it'd get him another $100m. But, imo, he'd never do it because he knows he'd lose. And Mayweather is too smart to get involved in that.