Don't a lot of welfare payments have strings attached, with regard to what you can spend it on? UBI explicitly does not, so you can use it for anything you want. If UBI did completely replace the social safety net, and someone gambles/fritters away their basic income and can't get a job to make more, are they allowed to starve (or depend completely on private charity)?
As everyone gets UBI, at first glance it might seem that the poor will get a smaller piece of the cake, but in reality a person with a good income will just give it back through taxes.
Of course, some specific people might end up being worse off (like a person who gets $30k of welfare), but on average poor people should be better off under UBI. Mainly because they can work and not lose anything.
UBI burden assumes current costs. That's short sighted.
--
Transitioning to single payer (medicare for all) would 1/2 our costs, at least. This is a no brainer, comparing Canada & USA's per capita spending.
I believe, but cannot prove, that adopting capitation model (incentivize wellness vs fee for service, treating disease) would knockoff another 1/2.
I think we'll still pay more than other countries, because we fund so much research, use some much more new tech. Which I wholeheartedly support.
--
Food costs can be brought way down thru better logistics, incentives, thereby reducing food waste. Ending subsidies for corn, wheat, soya, diary would reduce food waste and improve public health. A double win.
--
The USA has more housing than people. If people didn't need to relocate for jobs, allowing people to go to the available housing, per capita spending would also plummet.
3 trillion isn't anywhere close to the actual cost when you consider this.
This is explicitly one of the goals of replacing the current social safety nets with UBI.
UBI can't replace social safety nets. It's impossible. 12k for every adult is 3 trillion dollars, which almost always assumes cutting other programs. What about healthcare? I bet most would rather have comprehensive health coverage instead of 12k every year, but you can't have both. In the US we have neither.
You'd have to lose safety nets to make UBI work, financially. If you can show otherwise, do you mind explaining it?