It’s also worth pointing out that Skidmore’s total covers 17 years (1998 to 2015), whereas the Urban Institute’s $32 trillion estimate for Sanders’s Medicare plan covers 10 years. So the two numbers are not apples-to-apples to begin with.
Let’s put $21 trillion in context. The entire national debt is $21.8 trillion. According to the Congressional Budget Office, total defense spending from 1998 to 2015 was nearly $9 trillion. The CBO estimates $7 trillion in defense spending from 2019 to 2028.
In other words, completely defunding the military for the next decade would yield only one-fifth of $32 trillion. That’s a much better way of illustrating the cost of Medicare-for-All.