The annual minimum wage (at the federal level, not counting states with higher) is around $15k. There are about 267 million adults in the US.

That is double current federal and state welfare spending.

I'm dead tired right now so I'm sure I'm missing something, but considering that is far below the poverty threshold in any big city, I dont think we'll be solving anything by eliminating welfare in favor of UBI.

UBI is basically of no benefit to the upper middle class or wealthy, and it won't be enough for the poor who cannot work enough. It really only benefits the upper lower class and lower middle class the most.