The idea that technological society can't be maintained under a feudal system needs to be investigated further.

In the feudalism of the middle ages, while it's true that technology didn't develop, there was a class of scribes who maintained the accumulated written knowledge. And there was a civil engineering class that specialized in technically complex cathedrals.

One must also ask what all of the future 10 billion people are going to do. Make-work is one good answer, and a feudal system can supply that through a complex and superfluous hierarchy of servants and minions, each with a specified rank and privilege. The railroad system in India seems to have that system figured out, as do most military establishments.

And who says that feudalism needs to be unpleasant? If it can supply a basic level of physical comfort, plus football and gossip, 90% of humanity will be perfectly happy. It's only the troublemakers who worry about social justice who get upset when the hierarchy isn't "fair."

The trick is for the ruling class to manage the system so that there's no revolution. With the lower classes enthusiastically supporting the current system, we're quite a ways from that sort of bother...