One of the best-known advocates of UBI/negative income tax was Milton Friedman, and he most definitely was a libertarian.
There's a difference between idealistic versions of political philosophies that don't match human behavior, and practical versions of those philosophies that do.
Let's say we wind up with 50 million people who have no marketable skill. What's your solution? Let them starve? What are you going to do when they take exception to that?
If your answer is to simply kill them, good enough, but you need to face up to the fact that this is what you're advocating.
Jailing them is not an option -- that will likely cost more than the UBI scheme, and you can't even put them to work (it being a given that they can't perform any job that provides a net economic gain).
Second, this doomsday scenario of 50 million people waking up one day to discover there isn't a single thing in the world they can do to feed themselves is such hyperbolic nonsense you might as well be asking how opponents of a UBI plan to deal with a meteor demolishing Texas. Economies change. Sometimes a profession becomes obsolete. The people involved in it go elsewhere and on the whole everyone gets richer. It may sound unpleasant but it's only the same thing mankind has done a thousand times over already with great success. Unless you're mad about the fact that you don't have to milk your own cows anymore. This supposed choice between killing, jailing, or subsidizing millions of people is a false dichotomy that would sound like a joke to me if so many weren't sincerely buying into it. The other and far more reasonable option is to let free human beings navigate a changing world by adapting in the way each of them sees fit. They do it all the time. They don't need some savior swooping in with armloads of other people's money.
Free money to all citizens seems like a bold economic policy with no attainable goal and presumably would draw backlash from wealthy business owners who would simply raise prices on basic goods.
Instead why not just invest in public infrastructure like transportation and public housing, and education?
Why don't greedy people raise prices already today?
This includes everything from manicurists to medical doctors. Repealing these would do two things. First, it would eliminate an unnecessary barrier of entry into a wide variety of occupations, unleashing a great many new job opportunities. Second, it would give consumers -- particularly the poor -- access to low-cost alternatives that today are illegal. Government approval is no guarantee of quality anyway. We don't need it.
I would also suggest eliminating the Federal Reserve System and its monetary policy of gradual inflation. America's period of greatest economic growth was between the end of the Civil War and the start of World War I. Under a regime of moderate deflation the economy grew 4% year-over-year for a sustained period. Living standards rose across the board. Economic mobility was high. I believe this to be directly attributable to the incentives that exist under a monetary policy of slow deflation: saving is rewarded, prices fall over time, yet innovation and wise investment are greatly rewarded. (Gradual inflation, on the other hand, impels people to borrow and borrow a lot, which just on its own reduces one's economic mobility. Excessive borrowing can also be understood as consuming too much in the present at the cost of the future, which is where the boom-bust cycle comes from. Gradual inflation also makes saving extremely difficult. Anyone who doesn't want their savings to erode over time is forced to become a speculator. It subsidizes Wall Street at the expense of every holder of Dollars.)
Yeah, and? He thought it was the best solution to the problem. See above about the difference between political theory and political reality.
Second, this doomsday scenario of 50 million people waking up one day to discover..
Don't give yourself a hernia setting up those straw men. No one said anything about people "waking up one day" to find themselves unemployable. The discussion is quite clearly about a problem that develops in the long term.
There does happen to be historical precedent for this in the many other technological revolutions modern industry has seen. Can you find one that resulted in one seventh of the country just sitting on their hands strictly because they had nothing to contribute (and not because government "safety nets" paid them to do so)?
Education/training can't solve the fundamental problem, which is that a large number of these folks just aren't capable of becoming programmers, or surgeons, or lawyers, or anything else that of that nature.
What should we do with them? The problem isn't going to go away.
And we don't need endless Neurosurgeons; we need folks who can operate our sophisticated society efficiently and pleasantly. There are a million ways to live better, than being sentenced to a life of burger flipping.
You?
I notice that you don't mention any of these "million ways to live better". Do you have any specifics?
Better ways to live: Shopkeeper. Mechanic. Concierge. Furniture repair. Pet groomer. Now you try to add some more.