I thought the idea was that, firstly, current welfare already does this and UBI just gets rid of most of the harmful distortionary effects, and secondly, that UBI inherently must change the distribution of spending, and therefore while it will result in a long term increase in demand for the items poor people buy, that demand will eventually be followed by some increase in supply as the economy retools to take advantage of the new flow of money.

Would anyone be able to explain why this just-so story is wrong? My econ isn't great.