Money is a placeholder for value, not just paper. Like the "free energy machine" that's plugged into the wall socket: the power that flows from the wall socket is what drives it.
> The premise of UBI is that it creates a more productive society. If there is no possibility to be self-sustained, why implement it?
There is a possibility, there's a possibility for humans evolving telepathy. It's just not a certainty, and the probability isn't too high. But it's easy to test: get a bunch of people together that believe in it, make an agreement to pool some amount of resources and then pay out a UBI to each person. If it raises productivity, it's a big win for anyone involved.
If "free grain" is a "UBI society", then we're far beyond UBI already. In most of Europe, you'll receive an apartment, food, clothes, utilities, transportation, tv etc. That's 2020 reality. As far as I understand, the point of UBI is to go beyond that because these basics are seen as limiting.
> With UBI, people stop worrying.
Primarily, they stop working. We already have too many who are quite alright with the welfare they receive and prefer not to work. I really don't want to expand the number, because it just increases the work load for the rest.