The idea of a cash payment from the government to replace all welfare programs might be a great idea if…always a big if, if it replaced other forms of welfare. A cash payment would eliminate a lot of bureaucracy that currently eats up most of every welfare dollar spent. It would be fiscally wise to do this…if it worked. But it won’t. It will never replace other welfare programs but will simply become another monster bureaucracy eating away at the prosperity of the people who have to pick up the tab for it.
Milton Friedman, Friedrich Hayek, and Charles Murray all at time proposed some form of guaranteed minimum income to all Americans, but later changed their minds when they became convinced the only thing that would happen would be another layer of government welfare on top of all others.
Proposals to replace the income tax with a national sales tax are unwise for the same reasons. The so-called “fair tax” would never take the place of the income tax which would remain intact. The fair tax would just be an additional tax sucking the vitality out of economic growth and prosperity.
A good article with more about this is Time For a Guaranteed Income? by Veronique de Rugy.