Rome needed continual conquests to fund this benefit for its citizens. Margaret Thatcher said it well for governments that try to do this, "eventually you run out of other people's money."
Rome was also before the industrial revolution turned farming into a 1% of the population thing.
Whether we're before or after the point every human could be supported such they are happy and healthy, with more or less organization and/or equality, I don't know.
But I think it's a tremendous error to think that that point can not exist (is technologically and societally infeasible on a planet we'd recognize).
The difference is that we're dealing here with something that's more abstract than material goods (dollar signs and zeroes). In my view, this makes it possible, though exceedingly difficult, and I'm not sure how sustainable it would be over long periods of time. However, if the Romans could pull it off with grain over several centuries, who's to say we can't do it with money (which, in spite of its complexities, is much less fickle than grain)?
With that, the only addendum I'd propose is "unless you're very clever."
I'm not sure if we're clever enough, yet. But the Federal Reserve and the global financial system have come very far within our lifetimes, so it's not unfeasible to imagine us getting there one day.
