What happened was the United States became, for all intents and purposes, post-scarcity. We had enough stuff, now it was a game of who got it.

"Feudal" approaches to running corporations and is because Democracy isn't about being the most competitive, so of course it isn't right for an economically incentivized business. It is making an intentional trade-off from efficiency for long-run stability and human dignity. Feudalism is playing a finite game (you must win). Democracy is trying to play an infinite one (you must make it to the next turn).

It's the game theory playing on individuals' personal interests. The feudal system is aimed at assuring the status-quo for each participant in the power pyramid. The king relies on the high nobles to keep him and themselves in power, the high nobles rely on lesser nobles in the same way, and so on, everyone having to acknowledge and pledge loyalty/protection to someone else in order to get a modicum of security for their stance. Of course, everyone also has to extract and share resources upward. The collective interest is only secondary and this fact, for better or worse, keeps the life simple and fits the human psychic very well.

However, I do not agree that feudal social order is efficient. The nature of relations between people made things very transactional and specific, which imposed hard limits on the amount and degree of mobilizing and engaging people for pretty much anything.

Normally, that would also make it easier to model, but feudalism also tends to gain entropy through inbreeding.

I agree that it has less productive efficiency - democracies usually have stronger economies, often much stronger, than feudal societies. I suspect that this is actually related to the previous point. The increased decision-making efficiency of feudalism comes from a reduced need to get buy-in from people, but if you don't have buy-in from the people doing the work, they will probably half-ass the work. You see this throughout history: the serf works less than the freeholding peasant, the feudal society does not innovate, new inventions get shot down by the social hierarchy, industrialism does not take hold or when it does it's in inefficient top-down forms, etc.

There's still an unresolved contradiction here in that this would imply that feudalism would be more successful in times of quick change, but the historical record is that feudalism becomes very entrenched during times of stasis or decline, but often gets outcompeted in times of rapid growth and innovation. I still have no idea what's up here; perhaps it has to do with the existence of feudal-structure organizations (eg. corporations) within a democratic framework.