feudalisms end

 Feudalism was an incredibly stable system, until the final victorious warlord reached his final geographical conclusion. The natural defensible limits of his kinngdom. Once the threat of the tribe over the hill, or the tribe over the hill and the other 10 tribes they've conquered/married into, turning up and killing and enslaving your family becomes more theoretical the contradictions begin to pile up. No one in the dark ages is writing the prince. 

Once the final warlord secures the kingdom things have to unfold how they did historically. Increasingly lavish and sophisticated displays of class distinction. Imperial adventures and imperial wars, which are different in character from the feudal warfare that preceded them, in that they are not existential for the kingdom undertaking them and can be judged in terms of "good governance" by the growing middle class and politicking aristocracy of the imperial city, which is open to challenge and questioning, in a way that can end the basis of the feudal system in one foul swoop in a way that Warrick of Wessex getting knifed by his brother Stephen the Stupid never could, especially when there are 3 other kingdoms across the river ready to swoop in and invade the moment that happens.

On top of that, how does the king defend these large but stable holdings? How are the outposts of empire defended? In ways open to a caesar. And that caesars commitment to feudalism Once he has the kingdom he's free to remake it in his image, i.e. Cromwell or under slightly different circumstances Washington.

The real wildcard of course is technology. Maybe in a world without gunpowder feudalism lasts, kingdoms can't consolidate because knights, with their limitations, and the logistics of raising units of them, remain the dominant force on the battlefield. Maybe in a world without gunpowder empire building is too hard. That's getting into "is stable on paper" territory though. Banks were probably ultimately more fatal in the end. The ability for people who didn't have a fortress to protect it to amass wealth and not have to bury it in riverbanks or spend it all so you don't have much to take in your hut brought about a class mobility. The ability to borrow money was attractive to the aristocracy. The ability to loan money and amass amount of wealth that could provide loans to the monarchy allowed the circumvention of feudal patronage. 

In some ways it might come down to feudalism (as opposed to aristocracy) being non ideological or at least not a pointed ideological project , so the changes in social relations brought about by consolidated kingdoms weren't foreseen or  actively opposed when it might have mattered. A knight can't argue with a bullet and if your explorers are bringing in shipfuls of gold from the new world then what's your incentive to oppose mercantilism? Aristocrats are still on top right. That's all that matters.