This makes no sense. Fighting and grappling makes you extremely fatigued. Do you think Messi or Federer would be any less tired? Pacquiao is my favorite boxer of all time but all he has to do is box. He doesn't have to shoot/defend takedowns, grapple on the floor, get kicked in the legs, etc. You cannot compare any other sport to MMA. Just because they have good cardio in their sport does not mean it would transfer to MMA.
I agree with a lot of what the OP says. Fighters are the real A level athletes. We might not be as big as NFL guys but that is because we do too much cardio every day. Start training JJ Watt MMA every day and let's see if he can maintain all that muscle mass while also building cardio and developing skills. Because if you train martial arts six or seven days a week it is going to be very hard to also maintain that muscle mass and NFL combine numbers that everybody loves to talk about