You're assuming that the remaining $4trillion is lost. It's not. It goes to other people who will spend it on other things worth, in total, $4tril... even though some of them may need it less.
The benefit they derive from the increased flexibility that $20k could easily exceed $20k by itself - you've probably seen the research on how people make bad decisions when they are financially unstable. And all that benefit is raw positive.
For a BI to be bad from the perspective you're talking about it has to actively make people less productive. Simply giving them $20k doesn't count as a 'cost' in itself - it all comes full circle.
I'm sure you didn't mean this, but let's be clear the 6T is not returned to the tax payer that stumped it. Since this isn't generated wealth, just redistributed, it's a cost to someone. Someone's getting 20k, and someone's paying for it. And some people are going to be paying a lot more than 20k, and some people won't be paying in anything.
The benefit they derive from the increased flexibility that $20k could easily exceed $20k by itself - you've probably seen the research on how people make bad decisions when they are financially unstable. And all that benefit is raw positive.
For a BI to be bad from the perspective you're talking about it has to actively make people less productive. Simply giving them $20k doesn't count as a 'cost' in itself - it all comes full circle.