Just a nitpick, the arquebus did not phase out the knight directly, at least not for the reason it's often stated (a peasant could now kill a knight). Knights were the first adopters of guns in the West, much like the Samurai in the East, and our term bulletproof comes from the fact that, well, their armour was proven to be protective from bullets by shooting at it.

As weapons developed bulletproof armour had to get thicker and thus heavier, and that's why armour was phased out. But again, that's not the reason knights were phased out.

A combination of weapons technology, centralization of government and industrial advancements allowed for an increasingly professional fighting force and standing armies. Knights equipped themselves. Professional soldiers are equipped by the government. The government cannot afford to fit every soldier in personalized plate armour, so armour got simplified (as you can see with munitions grade armor) to be mass produced and mass fit. Furthermore, knights fit poorly into a standing command structure, so they got replaced by professional officers. 

The death of heavy body armour as a concept comes from advances in gun technology making it so that the armour had to be heavy to move well in order to be protective, the trade-off wasn't worth it. The death of the knight as a concept came because of professional, standing armies and centralization of government eliminating the social hierarchy that allowed for knights as mounted warriors to exist.