I think perhaps you underestimate the extent to which this change to mass armies has been driven precisely by the advent of firearms. I think it no coincidence that mass suffrage began to appear in the world at about the same time as cheap firearms. Prior to the advent of mass-produced guns, warfare relied on a specialist warrior class who essentially spent a lifetime mastering the sword. Feudalism was very explicitly based on this reality, where the warrior class held the power in society literally in the family.

With the advent of cheap firearms, it was possible to take the great unwashed, put him in a field and give him a gun that could kill his opposite number from tens of metres away. Suddenly war could be decided by which country could best mobilise its masses and put them in the field with mere weeks or months of drilling. With that reality some power inevitably devolved to the res insanitas, and the warrior class diminished in power. It was necessary to appease the masses either with national populism or by giving them a say in the polity. After all, an unwashed mass returning from war could still wield a gun if not compensated in some way.

The advent of mass firepower is also the advent of mass armies, and hence mass discontent. Guns changed everything.