Most jobs that people have now aren't strictly necessary. If everyone gave up their leisure time to do all of their own cooking and cleaning, a massive set of jobs would disappear overnight. If we all at local, another. And the list goes on. Effectively, what happens is, we invent new jobs that are based on want as opposed to need. It's easy to look at that picture and say 'there is so much unnecessary work that happens!' but really, that X% of the economy based on want is required for price discovery of the rest. That is what your last paragraph is touching on, how do we decide who does what and how much? In capitalism, the answer is price discovery, and for that to work, everyone has to participate in the market.
This is precisely what I'm getting at. We've built a system on the assumption of a forty-hour full-time work week, with a goal of minimizing unemployment and (recently) maximizing labor market participation. We did this because we need participation in the market for effective resource distribution.
What happens when our economy simply does not need that much labor to produce all the economic output we want? Do we force people to continue participating in the labor market in makework jobs just so that they can have access to resources? To what extent are we already doing this?
But logically it would seem that should be: Everyone still has to work, but a little less than hitherto.
Now the question becomes, how much less?
I believe the transition to post-scarcity will be gradual, that it will happen faster in some sectors of society than others, and that this process may take multiple generations to complete. This has been the case with every other major economic shift, why would it not be the case with post-scarcity?
If this is how the transition to post-scarcity occurs, societies will eventually hit a point at which many people still need to work to produce our economic output, but far fewer than currently do. How will our economic system handle that situation? The selling of labor is the primary mechanism by which we distribute resources, but if we get to a point where only 20% of the adult population are needed in the labor force then how do we distribute resources to the remaining 80%?
Now, where things get really interesting is that "needed in the labor force" is not a binary thing, either. What if there's a big slice of the population from whom we only need five hours of labor a week? And they coexist alongside another slice of the population from whom we need forty hours of labor a week?
Maybe we're already a few steps down this road.