Someone should tell that to all the MMA veterans who still get damaged by them. Hell, McGregor himself used them to great effect against a seasoned fighter like Diaz, let alone a rookie to the sport like Mayweather.
As for takedowns, McGregor was repeatedly taken down by Mendes, lost on the ground to Diaz the first time around and had to show strong takedown defence in the second Diaz fight as I remember it, especially against the cage, else he would have been taken down there too. Mayweather would have been quickly beaten on the ground in all three fights. Conor (weak though he is on the ground) still has years and years of training his ground game over Mayweather. If anything he proves that it takes time to build up that skillset, something Mayweather would have to do in a few months.
Remember CM Punk's attenpt at becoming a UFC fighter? It would have taken a lot more than adding Mayweather's boxing ability to his skillset to save him and that was after much more practice than Floyd would have and against a far weaker opponent.