[Disclosure: I don't actually know that much about European feudalism. Most of my posts contain a fair bit of speculative guesswork and imagination, but after finishing this I felt compelled to make clear it's almost all conjecture. Rather than make every sentence wishy-washy with too many qualifiers, I've kept many of the probably-too-firm sentences but am putting this here to qualify all of them.]
Under European feudalism, normative status hierarchies seem to have been relatively well aligned with objective character qualities and community contributions. For instance, the Lord organized, commanded, and actually fought with the army that protected the patch from external threats, thus earning the premium of admiration and respect (not to mention money) associated with his title. Social facts (the codified power distribution, titles and so on) and social values (what and who gets counted as good), seem to have been more tightly correlated than today, with both relatively well calibrated to their proper objective referents.
On the lower end of the status hierarchy, the most hard-working, responsible, patient, loyal, Serfs who cared about their family's future (all normatively positive descriptors) could save money and eventually become freemen — if they were blessed with the abilities necessary to do so. The epoch's techno-scientific inability to distinguish between inherited abilities and the above-listed character virtues was unfortunate and certainly caused much measurement error (giving too much or too little normative credit to individuals for inherited traits), but — discounting for their ignorance on these matters — the relationship between social facts and social values had to be better calibrated than it would be when mass-broadcast deceptiveness becomes possible. Drunks and brawlers presumably did not transcend their bondage, and those who avoided drinking and fighting would be more likely to gain independence. In short, good adjectives were likely applied to those producing objectively pro-social and self-rewarding effects, and bad adjectives were likely applied to those producing objectively anti-social and self-destructive effects. At least, I would infer, more so than today, when objectively bad people sometimes earn positive admiration from millions, and objectively good people sometimes receive nothing but punishment. Obviously, I'm being highly simplistic; ye old manor was no rose garden. But while there was much natural suffering and tragedy, and many typical human pathologies, it seems true that the social calibration of normative worthiness with its objective empirical referents was far less vulnerable to the kind of systematic, impervious-to-error-correction divergence dynamics we appear to be living through today.
The philosophical and behavioral backbone of the post-Roman patchwork was Catholicism. It stands to reason that this was the unique condition that allowed a high degree of fragmentation and decentralization, but nonetheless a high degree of shared identity, meaning, purpose (relative to anything we know today, anyway).
The feudal community codifies objectively existing differences in human temperament and ability, which may be natural and hardwired or arbitrary and unjust, but — with the surplus of social goodwill produced by this factual and spiritual attunement — the powerful are genuinely invested in lifting the floor of their most downtrodden subjects. The weak also are genuinely invested in — sincerely praying for, or "rooting for" (to use the contemporary term for "prayer") — the success of their Lord and his army. European feudalism therefore provides some historical evidence for the consistency of what I have previously theorized as noble communism, and it suggests that the Catholic faith may be uniquely effective in solving the coordination problems of the ideal communist model. It was the Catholic faith alone that sustained cohesion, meaning, and collective economic productivity (though admittedly not optimality) in a context that was fundamentally libertarian. It was also unique to this Catholic patchwork that only from here would we observe the intelligence explosion that we now think of as modern capitalism.
One should not ask why the feudal commune failed: it was a genuinely free communism that succeeded in generating growth behavior (to be called capitalism). It was probably only in this fragmented but high-trust context that capitalism could emerge.
In its first few hundred years, it's been like an angry tiger just released from an all-too-small cage, but on the world-historical timeline a few hundred years is nothing.
The pro-growth, libertarian Catholic communism of European feudalism was so successful that for a short period of time, several proud and arrogant generations thought they could do away with God. They tried, life eventually became unbearably empty, at the same time that scientific rationality now affirms the likelihood of a creator God at the beginning of our time, and a second coming of God in the near future. All of this is now being realized, and a return to Catholic communism may be the only path forward, on rationalist grounds, aesthetic grounds, and ethical grounds.
Another virtue of Catholic communism was that it sustained itself for hundreds of years without any race consciousness, which had not yet been invented in its modern sense. Thus the Catholic communist model offers a viable and much better alternative to ethnic identity as a principle of cohesion in the West.
So the question is not "How could we make feudalism work today, if it couldn't work in the past?" It did work in the past, all too well! The question is rather whether secular capitalism can last, for more than a few hundred years. Feudal communism worked for hundreds of years and is generative of capitalism; secular rational capitalism has worked for a few hundred years, but its rapidly being looted by rent-seekers within and anti-western, anti-secular enemies on the outside. The human biomass that is now merely a plaything of the rational secular capitalist super-system has no will to fight for anything other than its own resentments. Any sufficiently aggressive and repressive force on the inside or outside of secular capitalism may very well destroy everything before artificial superintelligence takeoff locks in.
It is not for nothing that the threat of militant Islam again rears its head today, right when the decline of Catholic communist Europe is approaching completion. The reason why there was not much innovative art and culture in the Dark Ages is because most of the human effort went towards the military defense of the Catholic patches against pagan pirates from the north and Islam from the south. It stands to reason that Catholic communism was adaptive for keeping out regressive militant Islam, and the threat of progressive militant Islam incentivized the Catholic communism. We are only being reminded of this today after a long hiatus of lazy arrogance; that if a dignified and meaningful life is not provided to all by the noble, then the children of Europe will sooner join the Islamic holy war than resist it. And most of them will be indifferent at best.
We are well aware of the ways in which secular communism is typically not a stable game-theoretic equilibrium. We have learned this through many data points. But we are less aware of whether secular capitalism is a stable equilibrium, because it’s a unique world-system experiment with an n = 1. So far though, it's not looking good if you ask me.