Good point.
That is why I said "solid UBI", as in more than merely survival wages, i.e., enough to not merely buy food & shelter, but also to live.
That said, this does need some thinking through multiple stages. On one hand, societies did still work when there was effectively unlimited slave labor, but that may be no more than a rough proxy.
Go to the endpoint assumption that AI and robots can produce everything needed for the population to live, and they are owned by 1% of the population. They made so much money so fast that they bought all the means of production. As of 01-January-2025 everyone is fired. Now what? Your'e right, no one can buy anything. The remaining populace cannot do anything because the new oligarchs have enough money and power to buy &/or threaten any politician.
The population overall is not going to simply lie down and die. About four days after running our of all the food in their pantries, they'll be revolting in the streets. One plausible result is a lot of carnage and the oligarchs are all killed and the AI is destroyed and outlawed. Or, they actually have sufficient command of the military and no defections and the military isn't smart enough to figure out that they're next on the starvation list, so the populace is wiped out as they revolt, and the world is left with the 1% of owners, and 1% of military. Or, there's some kind of balance reached, and the non-AI-owning class of fired people reconstitutes something similar to last year's economy, while the 1% go off to Mars or withdraw into their metaverse-ish thing.
That's just a few random thoughts on rolling the dice among the big forces, but it never plays out like that, so all of these are 99%+ likely to be wrong.
Seems like the only thing we know is that this potentially massively magnifies instability.