Being prosperous isn't as simple as printing pieces of paper, obviously, but if you automate all the work which is required of people, then they will be prosperous without having to lift a finger. If all the gains from automation only go to those who had enough money to buy the machines which now do the work, you're going to have a huge chunk of the population who are desperately poor, and I can't get behind that at all. Society will need to find a fair way to distribute the "wealth", and UBI is an attempt at that.
I know I'm probably on the wrong website to promote state solutions to our troubles, but the free market solution is bound to end up with unelected people with just as much power as the state currently has... the rich will get richer and more powerful. I've read plenty on the subject, from philosophers and economists, and I just can't see it ending well.