I'm going to cover a few topics here, and I'm sorry to simply come out and say "you're wrong", but it has to be done. (I'm covering the middle to late Medieval period here) WALL OF TEXT
Firstly, armour usage: Armour was relatively cheap, and munitions harnesses (which was the 'standard' kit of a footsoldier, consisting of a breast, back and arms, sometimes with chain underneath) made up about 90% of armour sold. The whole kit would cost around 12 shillings, which was a month's pay for a footsoldier, and less for a man at arms or sergeant. Helmets and weapons varied, but again, perfectly useable variations were available for around a week's wages.
The important thing for middle-late medieval warfare is livery and maintenance, which is where you would wear a lord's livery (colours) and follow him, and he would pay for your food and equipment (maintenance). Lords would buy armour and weapons in bulk for their troops, which obviously could get expensive. The thing is, it happened despite the cost as soon as the longbow became widely used. If you turn up with a bunch of peasants, archers will slaughter them en masse, or armoured troops will massacre them. Also, if you take your peasants to war, there is nobody taking in the harvest. Because of the cost and the risk, you ended up with small, professional armies. To reiterate an important point, pretty much all troops, even archers, had munitions harnesses, but only the "fancy" armour has really survived, so that is what we think of. They were also the closest thing the period had to a modern workshop, where numerous shops would put out thousands of pieces.
The next point is full plate armour. Knights and nobility began to really use it in the thirteenth century, but it was so expensive that the poorer squires chose not to be knighted because they couldn't afford the armour. A few people had it before them, and you will see them in tournaments, where its early efficacy is doubtful, because you constantly hear things like "the earl of x was gravely injured and died a month later from his wounds" and "x had his armour hacked from his body". Bear in mind this is with blunted weapons. As the armour became cheaper and more effective due to things like fluting, it was more widely used, and people stopped using shields, which slowed you down and weren't effective at all. What we think of as shield fighting is completely wrong, as shields were used for deflection, which is why they were curved. Knights ended up using their arms and hands to fend off blows.
Which brings me to the final point, the mobility of full plate. Lots of people talk about weight, and about how replica armour can't give you the full picture. I invite you to find a video from the curator of the wallace collection, who (unfortunately) took out an authentic suit of armour from a king, and wore it, filming a few videos of him sprinting, doing forward rolls and getting up off his back. Full plate, towards the late medieval period, made almost no difference to mobility, and made people very hard to kill. However, it could be done, as evidenced by a few sources, including one from the wars of the roses, where a displeased noble killed his captain in one blow from a warhammer.