Let's grant the premise. UBI, significant enough to live well on, luxury cars and dinner parties for all.

Who sets the amount? Who controls the infrastructure producing the unlimited resources? What happens when you vote the wrong way, or protest the wrong policy, or simply become inconvenient?

A population with no economic function has no leverage with which to resist a reduction, a condition, or a withdrawal. You're describing a world where 99% of the people are entirely dependent on the goodwill of whoever owns the machines, and you're treating that goodwill as an unchanging variable. The history of every human institution suggests that power without accountability eventually behaves like...power without accountability. Even assuming the benevolence of the people holding all the cards isn't naive optimism, it's the same mistake that makes people say real communism just hasn't been tried yet.