Mayweather's point is he can make his money fighting anybody. McGregor can only make this kind of money fighting Mayweather, even if it is 1/7th or whatever. And he's probably right. McGregor should do it, but it will never happen.
Yes, in a straight boxing match Mayweather would school McGregor (not sure he could knock him out, but would be dominant for 10/12 rounds.) I think actual MMA fans understand this and wouldn't hold a loss of this nature (not his sport against arguably the best in THAT sport) against him, so go get paid. In MMA, you can lose at anytime anyway and possibly hurt your name/brand, might as well go collect a MASSIVE check even if a loss is almost guaranteed, since there isn't that much downside.