Those aren't the only choices though. One is that you just accept that there will be a large number of unemployed, and account for that by having a basic income guarantee. Another would be creating make-work for many people. And there are others, it's not just "go back to 1970 or birth control".
If you accept the idea that 70% (and rising) of the population is "useless" (presumably from only an economic view), then the idea of a basic income guarantee becomes difficult to fund I would think.
If you accept the idea that currently 70% of the population is useless, then it's obvious that right now 30% or less of the population can easily provide sustenance and basic income for everyone; and the expectation is that increasing automation will drive that number much lower.
The optimistic scenario is that 'soon' (a) 10-20% population working at 20hours per week can produce enough "stuff" to sustain everyone; and (b) there is enough population who would actually do that even if they don't have to - because the work gives them fun, meaning and motivation.
If right today everybody was given really good basic income, most would take a long vacation from their jobs - but quite a few would go back to productive work afterwards; not at 80 hour weeks but at a reasonable amount, and it might just be productive enough to sustain the world.
And if everybody was given a 'sustenance' basic income, then almost everyone would still be motivated to work for a slightly better lifestyle and luxuries. Again, it wouldn't motivate people to juggle two jobs with raising kids; but we don't actually need anyone to do that, we have more workers than we need anyway.
The result of the 20% 80% situation becomes of course, that there is one tactic that the 20% can use to increase their economic output five-fold (and it is not likely that multiple such tactics are available to them) ... And the 80% get the same impulse to kill the 20% : after all they're the only ones who can increase their comfort, and they persistently refuse, so conflict is natural.
