I have an economics degree. I'm not claiming to be an economist here at all, but I'm mentioning it because it's relevant to what I'm about to say here: What I learned throughout my undergraduate education is that economics is a social science built upon the _presumption_ of scarcity.
Classical economics doesn't apply if you break that invariant. We need something else if we achieve post-scarcity, and I'm not sure that it'd be another existing mainstream economic school of thought.
Note that I'm not disagreeing with you - your point is valid. I'm agreeing on the "create artificial scarcity" response that you're observing. I suppose what I'm trying to say is, whatever other unfalsifiable assumptions you're pointing at in economics shouldn't matter because the broken scarcity invariant breaks the rest of it. Trying to use an economic argument in a post-scarcity scenario is, to me, applying the wrong tool for the job in order to preserve a specific interest. That's my opinion, anyway, and I'm sure someone else who has more experience or education in the field beyond my 4 year degree 20 years ago could tell me I'm wrong.