Political Memes

It would be awfully hard for a vassal landholder to get the kind of materials that they can’t produce on their own. Iron, tin, copper, lead; depending on the area, lumber, as well.

Yet most fiefs during the height of feudalism were autarkic, and engaged in minimal trade, much less redistribution from their overlord.

There was also the military presence keeping the brigandry in check (including from other feudal lords.)

Considering how rampant brigandry was, dunno how valid that is. As for other feudal lords, those are, of course, the peers they despise.

Vassals also exchanged military service for land. (And the serfs that came with it!)

Land could not simply be revoked in most feudal systems, though, and was more often inherited than granted by the overlord.

None of your points are wrong, necessarily, but I don't think they're major compared to the core pillar of "I trust my overlord to oppress me only a moderate amount, while I don't trust my neighbors not to oppress me a much greater amount, so I would like it very much if my overlord would just oppress everyone. I'll lick his boots for it." It becomes especially apparent in Bastard Feudalism of England and in the later feudal system of Japan in which land revenue, rather than land itself, was what was granted to many warrior-vassals.