Here’s the quick and dirty of the thing, beginning with a quote from Wallace’s letter.
The strongest arguments for universal income center on its elimination of work disincentives by the unconditional nature of such grants.
Leave aside the fact that free money is its own disincentive to work. A UBI will only increase demand—all that seemingly added money with which to buy stuff from necessities to goodies—without increasing supply. The resulting price inflation will very quickly reduce the buying power of the UBI to the same level that the current poverty-ridden man possesses. A UBI will not make anyone better off.
On the other hand, it will make everyone, including the poverty-ridden man this is intended to help, worse off: the UBI will come out of the pockets of everyone in the form of current taxes, future taxes to pay the borrowings, and/or devalued dollars. This will reduce monies for investment and innovation, truncating economic growth and reducing economic mobility. The latter will act to confine the poverty-ridden man to his poverty.