Regarding your second paragraph: You're looking at this as a measurable global issue that will happen at a single time resolvable by a single government entity, but that's not how it will unfold.
It won't happen overnight, so there will be no point for those options to be evaluated. Let me pitch you (what I see as) a more realistic option:
Some countries will try to implement UBI. They will encounter the same problems opponents have been warning about. The productive will (over time) move to countries that don't take so much of their wealth. The people receiving more than they're contributing will have more children, will have more votes, and will vote to take more and more from the decreasing minority. Even if you can make food truly post-scarcity, housing, healthcare, and other goods/services are not.
If everything is truly post-scarcity: It'll be no problem. Those countries won't really need the wealthy. But I'm not sure true post-scarcity actually exists, and I think (especially in the US) UBI would be a moving target that only ever goes in one direction.
You're right, though, that I don't really have a solution if (e.g.) 80% of people no longer have a meaningful way to contribute to society. That may be fundamentally impossible to solve. But that may never happen and we may never have true post-scarcity, so we shouldn't be redesigning society based on the premise that wealthy people are an endless money tree, and if it's wrong, society collapses.
When this situation comes, the good option is that the society simply gives the new class of unemployable people their basic needs one way (like UBI) or another; the evil option is that the society doesn't give them the basic needs and they starve (since their labor has no value and they have nothing to trade for these basic needs); and the third option is that they take the basic needs in a revolution that likely destroys half of the society. Really, noone wants the second or third options, including most of the 0.1%, so we will see some mass redistribution system being implemented.
In 1917, taking all the resources from rich people and giving it to poor people wasn't enough to keep the poor people out of starvation for long and destroyed the productivity totally. In 2017, taking more money from the rich to feed the poor doesn't require even close to taking all their resources, as feeding the poor is so much cheaper compared to 1917; it would be deeply unpopular but is doable without killing the rich, and the society could sustain such level of taxation indefinitely if they wished even if nothing changed - but it's going to change, and in 2037 taking enough resources from the rich (i.e. the owners of highly automated means of production) to feed everyone UBI-style would be expected to be just a minor tax, and a boost for the economy, giving spending power to people who'd otherwise have no spending power at all.