The only thing I have to say on the debate about what combat sport has the best athlets or the most fit people is impossible to say.
However to say MMA guys are lazy as they just lay on one another is rubbish yeah some fighters might do it from time to time but boxers can do the same just standing up clinching. Also you have to rememeber how tough it can be actually get the takedown in the first place as is holding them down there. MMA is more physically demanding I think, but not say boxing and kickboxing are easy as I do not think that for a second.