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.