What happens as more and more people decide to live on the dole? Are you going to force them to work? What happens when the last wealthy people you could tax into paying for it flee your country with their money?
Utopia fails because, "HUMAN."
Ceteris paribus, prices go up, and living exclusively on the unconditional basic income becomes less attractive. With a UBI, there's no reason not to work if you are unsatisfied with what you get from the UBI, since you don't lose subsidy for working. So, there's a natural negative feedback control system.
> What happens when the last wealthy people you could tax into paying for it flee your country with their money?
If people making profits in the country that could be taxed leave, then profit making opportunities open up that others could, and someone will, take advantage of.
You aren't going to defeat human nature.
Those theories have been around for hundreds (thousands) of years, but oddly, it never happens. There's a reason for that.
People are not automatons and they usually think for themselves (with varying degrees of success) and won't fall into line with the rules necessary to sustain a utopian society.
You'll be left with nothing but a gun in your hand trying to force your well upon them. Forced utopia begs for revolution.