As a kid, we had AC, but we played outside all day long in Mobile. You got thirsty, you turned on the water hose, let the hot water run out, got a drink and kept on playing.
I was thinking of my days in Mobile, playing basketball for 5 or 6 hours in a row, stopping only to get some fluids (occasionally). I think it's a little more complicated than that, and some of this is probably a bi-product of the extent in which we push the performance of these athletes. In my days of playing sports every day, often for hours, I was as thin as could be. I was muscle and bones, and I had more the build of a marathon runner or a bicyclist than of today's athletes. I didn't work out, why should I? Even at my slender size I was stronger than most people my age. My body was allowed to adjust to the conditions.
These athletes are so performance oriented, that it's not just natural conditioning, it's hours upon hours in the weight room, it's diets designed to put on mass, etc... and then when the coach runs those guys to death, it's going against a lot of what you were having them do in terms of putting on weight and muscle. I think some of the issues are born of on one hand pushing these guys to the extremes in terms of performance, then pushing them to extremes in terms of endurance.
Having played both Rugby and Football I can attest the football uniforms in order to afford you protection do NOT dissipate heat. Basically your cooking inside armor.
I don't get any of the rugby comparisons or how people got started on that. To put this issue in perspective, we are talking about putting someone in full pads, and making them run as much as possible, and then deliberately keeping them from leaving the game when they want to. Anyone who doesn't understand that's a recipe for disaster, or comes up with "Rugby!" as a response is being obtuse.