The feudal serfs did not possess over a hundred million modern firearms and plenty of ammunition.
The real determining factor in what political system you have, depends on how conflicts get resolved.
When it was a matter of which side could show up with the most men with swords, and the average man could afford a sword, then you could have Athenian democracy and the Roman Republic.
When professional fighters, wearing expensive armor, could defeat much larger groups of regular people, then you had the Roman Empire, and finally feudalism.
When cannon could knock down castle walls, the feudal barons had to obey their kings.
When rifles in the hands of common people could defeat professional armies, then you had the Republic of the United States.
“The feudal serfs did not possess over a hundred million modern firearms and plenty of ammunition.”
They are doing their best to rectify this, every day.