i read all replies and they were good. i'll add a little.
But, what happens when, say, manufacturing reaches near-total automation--will humans still find ever increasing ways of creating value for themselves?
yes. our desires will always provide new things to value.
I just mean, in theory/on paper, is it possible to have an economy that is purely service-based?
no, a closed economy always needs production. services are just a function of increasing wealth.
by a closed economy, i mean the world. could a country get by on a service economy? yes, but some country somewhere would have to produce something. the producers would have the highest standard of living and the servicers would have a lower standard even though there are many service jobs that pay more than producing jobs. still the producers would have the most benefit.