[CRConrad]

Yes, logically / mathematically it seems it must be: Those additional people also contribute their 40h/wk, so more work gets done in total. That should be sufficient to support even more more people, not just more people, if they're all also more productive.

Or, IOW, "productivity" is a per-capita measure, so the total number of people doesn't factor in. (Or perhaps rather, gets factored out?) Anyway: People becoming "more productive" means each one producing more per hour — or per 40 hours — than before, so it's weird that they still need to work just as much as before, only to maintain the standard of living they had before.

(Running faster and faster, just to stay where they are... Curiouser and curiouser, said Alice.)