I don’t have a strong opinion on UBI, but I want to challenge you on this.
If everyone gets $20k per year, let’s say the median earner goes from $40k to $60k. An increase of 50%. And let’s assume that we don’t increase taxes, even though it would definitely play a role.
Pretty soon, inflation kicks in and everything goes up 50% in price.
But a low income earner of $16k per year is now making $36k per year. More than double! So the 50% increase of prices means that the UBI helps the lower income person’s purchasing power to be on a more level field with the middle earners, even though the middle earners obviously still are better off with their larger income. Just less so.
And the person making $200k is now making $220k. But the 50% inflation means that they have to lower their consumption a bit, to be more in-line with the median earner.
So introducing UBI like this causes an automatic rebalancing between different levels of income, somewhat, even if the median earner is unaffected. Obviously this is greatly simplified though.