All basic income does is define a minimum possible level of income. And, for economic purposes it really doesn't matter whether you measure income in goods or currency (economic sustainability remains the same either way).

So what you're really saying is that it's not possible to guarantee any minimum level of living at all, at least not in a way that's economically sustainable. Conversely this would imply that a functioning economy requires some people to fail to reach any standard of living whatsoever (basically they die from lack of food, housing, medical care, etc.).

So sure, any level of basic income might be economically unsustainable, but if so then a functioning economy seems rather overrated.