i realize that the rest of your post details technological advancements that could create this condition (on an indefinite timeline), but you also seem to be saying that it is already possible today. i often see this sort of claim these days, and i always wonder why this is treated as self evident fact.
in 2015, the PPP adjusted GDP per capita over the entire planet was just under $16k [1]. this is only 125% of the US poverty line for a single person in the same year [2]. even if we could deliver this sum to every person in the world without losing production capacity and with no administrative cost, it hardly seems enough to provide what you mean by a "modest living" to everyone. much closer to mere subsistence in a developed nation.
[1] https://en.wikipedia.org/wiki/List_of_countries_by_GDP_(PPP)...
[2] https://www.census.gov/data/tables/time-series/demo/income-p...