You realize that's not true right? The advent of crossbows did not make armored knights obsolete, and in fact armored troops could have continued being used well into the are of firearms, (as the iron used was enough to block bullets if crafted well, let alone a few measly crossbow bolts) if not for the cost of maintaining such a force and the amount of necessary training. In fact, because your average Knight's armor was the highest quality available besides the king's himself, the only way a crossbowman with as little training as you describe would have been able to fell one is by landing a lucky shot in the chinks between the plates.