Guns punch right through plate armor
It's much quicker to train a gunman than use the English Longbow.
False, and true. Early firearms did not punch through plate (in fact, many surviving sets of armor from that period have visible "proving" marks where they were tested by firing a firearm at them and showing it would not penetrate). Instead, really heavy armor was a response to early firearms (you didn't need full plate to stop a sword or spear). Later, when the power and availability of firearms became more of a factor, they did become much easier to equip troops with (and far easier to train them in using). From that point forward, heavy armor became more trouble than it was worth...