You're forgetting about growth. Is UBI possible today? Probably not.
Quick googling (so take my numbers with a grain of salt but my point is to illustrate not be exact) says US economic growth the last 10 years has been ~45%. If that trend continues today's $3 trillion budget could be $4.5 trillion in 2030 - UBI starting to look much more possible. Another decade of 50% growth and the budget is $6.75 trillion in 2040 -- UBI seems absolutely possible. Sure there's population growth to consider as well, but you get my point where at some point the math works pretty well.
Quick googling (so take my numbers with a grain of salt but my point is to illustrate not be exact) says US economic growth the last 10 years has been ~45%. If that trend continues today's $3 trillion budget could be $4.5 trillion in 2030 - UBI starting to look much more possible. Another decade of 50% growth and the budget is $6.75 trillion in 2040 -- UBI seems absolutely possible. Sure there's population growth to consider as well, but you get my point where at some point the math works pretty well.