argument that "The user can (and should) turn from a reader to an editor, and vice versa, at any time." might sound good on paper, [...]
It doesn't really need more. My point is not necessarily that it's true, but only that it *must* be true, because the opposite would be the antithesis of what makes the Wikimedia projects exist. Call it a dogma, or a foundational myth, if you prefer.
put it is actually provides the ideological basis for unfettered site rule by a professional caste.
Curious prediction, but not seen in reality so far. If you really want to put it that way (caste vs. community), I'd argue that, against the "we represent the 500 millions readers" bulldozer, the community has a better weapon in "we are the 20 millions editors population" than in "we, the council of one thousand,* are the whole city".
Nemo
(*) Cf. e.g. https://en.wikipedia.org/wiki/Crotone#History (sorry, Polybius again).