But even toning it down a little, if anything really were unreasonably effective, wouldn't almost everyone be doing it already?

Having a AGI that goes FOOM and fulfills all of your orders is comparable to having a circle of infinitive wish spells. The fact that nobody yet has such an AGI doesn't mean that it's impossible.

Money just beats barter on pretty much every axis you can imagine. It is massively better than the alternative of a no-money society.

Societies without money aren't barter societies but gift giving societies. Money doesn't beat gift giving on every axis.