I clarified it only because I also thought you could think I was referring to all of the folks who have bikes in their garage with flat tires, derailleurs extremely out of whack, or other problems. I agree with most or all of what you said BTW. However, sitting in bleachers I may look much like the folks in your photo yet I'm comfortable riding all day on an upright.

Leaning forward creates skin folds which cannot naturally cool as exposed skin can. This causes a bit of extra heat throughout your body but also within the folds you overheat quite a bit. Also, when you sit upright you have greater 'wind exposure' vs being aerodynamic so upright may cool you better from this aspect as well.

Personally, I tend to also ride a bit harder on a road or hybrid than on a Dutchie.