It would be almost impossible to fund today, but might not be that hard when we finally need it.

If you figure $20k per year for 330M people, that’s $6.6 trillion. Total US GDP is $23T, so today it would be colossally expensive. But GDP grows 2% per year. At that rate, GDP will add an extra $6.6T in about ten years - and that’s after inflation.

So in theory we’ll have an extra $6.6T to play with in another decade. That doesn’t mean it still wouldn’t be difficult. But it’s feasible.