Two further points spring to my mind:

1) I thought the danger Archers posed to cavalry was more about the horse than the rider - an unhorsed rider in cavalry armour would not fare so well on foot and horses are harder(more costly) to armour.

2) The vast majority of troops were not as well armoured as knights -only a fraction of a contemporary army would have worn "white" armour, most of the troops would have had far less protection (quantity and quality).

These would mean that the fascination with longbows being a medieval/renaissance wonder-weapon are somewhat beside the point. The analysis of armour penetration etc should instead focus on the defense provided by horse armour or the more common human armour types -never mind realistic angles of attack.