That's what the experiment is for.

But let me put this bug in your ear:

Most economic models are based on the premise that people can walk away from a potential transaction with no losses. That when people trade value, each party leaves the table with more value than they brought to it but if they leave they are no worse off. That's a large part of the brilliance of the free market -- value is continually created.

However, this breaks down when one party loses value if they walk away, negative utility, especially if the other party knows it. And it is especially troublesome when that negative utility is expandable e.g. when the alternative is pain or death -- things people will generally pay whatever they can to avoid. That's why strong arming is not considered good economic policy. It can lead to net destruction of value. In some cases this can be ignored because the value created for one party more than makes up for the value destroyed for the other party but, in general, it is more than possible that the net effect is negative.

With the poor they might not be threatened directly but the consequences of not, say, selling their time for less than it is actually worth can be just as bad as if someone was robbing them.

We're not talking about making people rich, we are talking about removing desperation and avoidance of death and pain as a motivating factor.

An argument for implementing BI in the foreseeable future isn't really a "nobody should be forced to work" argument, as desirable as that might ultimately be. An argument for implementing BI in ones own developed country in the near term is a "nobody with the right passport should be forced to look for work" argument. I think social security - essentially insuring people from a risk of unemployment caused at least in part by policies enacted by an elected government - is a good idea, but I categorically don't agree that my fellow Brits should have an automatic right to receive more than the world average purchasing power adjusted income without even thinking about doing anything for it.

They may have done it mainly for the benefit of their own heirs and successors, but now that it's done, it's done. All those improvements multiply, and Brits get more because it takes less individual effort to produce the same amount in the heavily-improved UK than in a country where improvements are more easily destroyed or converted. So anyone willing to obey British law, participate in the British economy (accept and spend pounds), respect British infrastructure, and assimilate into British culture should get more than the rest of the world, because those are the practices that allow Britain to be more productive than the world median.

A hedgerow along a country lane does not grow that way naturally. It takes an effort to force it. But once established, it just needs a regular trim and an occasional re-laying to keep the livestock out of the road. You just don't need to pull a new barbed wire fence in front of a blackthorn hedge, so why would anyone complain that no one is doing it?

The resources of the country are preferentially distributed to its own citizens because one country lacks the sovereign authority to force any other country to change its culture and institutions to be more productive. If those other countries wanted to be as prosperous as Britain, they could fork the British system, or selectively merge changes from it into their own repositories. See also: USA, Canada, Australia, New Zealand.

So those people aren't exactly thoughtlessly doing nothing. They are being British, as hard as they possibly can. And that allows the UK to have nice things like the NHS. In contrast, Americans, in being as American as possible, can only manage a hybrid public-private healthcare monstrosity that no one can examine too closely or for too long, because most of our psychiatric hospitals are cleverly disguised as prisons.

In light of that, there's really only one essential element that needs to go along with open borders. Assimilate the immigrants.

Labour isn't labour when it's sitting with its feet up enjoying the privileges of accident of birth, and any benefits from their additional consumption that might trickle down to the developing world poor can easily be cancelled out if unconditional redistribution happens to have side effects on the Western economies in question. Not to mention the likelihood of barriers to entering Western countries getting a lot higher...

That isn't a remotely analogous situation. This is people not being able to afford food, suddenly being able to afford food. Being suddenly very rich brings with it opportunities and responsibilities that simply being fed do not.

That's a bad comparison, there have been studies that show a small no-strings-attached monthly allowance made people's lives easier and overall better. They did not spend the money on crap like most lottery winners do.

I assume the reason is that those people realize they're still poor, but got a bit of help. Lottery winners get more money than they can handle, start assuming they're rich and that messes up their spending habits.