Should AI and robots "take all the jobs", and most people lose their ability to make a living, they wouldn't have any money to pay for the goods and services thus created and would be permanently locked out of "the economy".

If enough penniless, unemployed people gather, one of them will offer to cut the other's hair in exchange for a repaired shoe or some eggs and since nobody has the cash to buy a robot to do the work, they'll just have to it themselves.

This spontaneous new economy could only be stopped by prohibiting people to engage in mutually beneficial exchanges and if that happens, you don't need UBI, you need a good, old-fashioned, 1776-style boogaloo.