Plate (and the ludicrously heavy weapons soldiers started weilding to pierce it) did, indeed, make soldiers tire quickly, though.
They did, but it also meant they didn't usually have to do things that an unarmored man had to do in order to protect himself from harm, and either one could be armed with the same equipment, with the armored man able to use a handier weapon. Shields began to disappear almost completely because armor provided such excellent protection against both ranged and hand attacks. The attacks it couldn't protect against (arbalests, heavy firearms), shields couldn't either, at least not without making them much too heavy to be carried by one arm all day.
Pollaxes and the like are, in fact, not all that heavy, but they
are less handy than a sword (but still effective in skilled hands), the weight being concentrated toward one end. Swords were lighter (weighing a pound or two, with wooden training versions—"wasters"—actually being
heavier), but could also be difficult to wield, depending on the length. Modern assault rifles, like the M16 (almost 9 pounds when loaded), are actually heavier than most of these weapons. Of course, you don't have to
swing an M16 around all day, but still: these weapons were not too heavy for the fighting man of the day to be comfortable with.
Horses and those charging tactics gave those knights the ability to quickly disengage, flip up their visors, and get a breather in between rounds of combat.
Yes, but that's assuming those knights were disciplined enough to pull back once it became necessary to do so (and not a moment too late) and to not to carry the fight too far. The worry was blowing their horses, which would have left them extremely vulnerable, especially to a counter-charge by fresh opposing cavalry. This happened to the British heavy cavalry at Waterloo. Neither the riders nor their horses were armored (although the French cuirassiers wore, appropriately enough, thick steel cuirasses and helmets). Whether armored or not, a cavalry charge has to be timed right and it has to have a clear objective, or the men could be left vulnerable.
Infantry fighting in formation would actually fight in teams that would specifically let a certain portion of them rest.
Never said otherwise, and this is an argument for the use of heavily-armored infantry, not one against it. They can pack tightly and enjoy protection from their armor, their fellows and, possibly, their shields, without worrying about dodging, and if their sword arms do become tired, they can retire to the rear and recover.
Well-formed heavy infantry in general are difficult to break. Light troops, not so much, but they have their uses, which, to get back on-topic, are not really well-reflected in DF at the moment.