I do agree that a graduated UBI (negative income tax) would be cleaner than the current welfare system. A smooth gradient out instead of a sharp cut in benefits. The incentives would align substantially for people seeking to escape the poverty trap.
A major issue for me when I think of this is the incentives for increasing the amount until it is unsustainable. Being able to vote yourself more money is… well. A ticket towards candidates promising to give people more money out of the pockets of others.
This would incentivize brain drain as well as immigration of people in dire straits. It would also incentivize a population boom since people would no longer worry as much about being able to support their family. This in turn makes the problem worse.
Although this applies to any kind of welfare. So it may be strictly easier legally to end the old programs and start on this one. The savings from no longer dealing with red tape, frustration, and bootstrapping from meagre resources may actually be sufficient to counterbalance these negative effects.
I do wonder on the effect of various UBI schemes on people’s productivity and life choices.