In other words, the US is "selling" at 5 times earnings.
For most of the history of the NYSE, average company stocks sold at at P/E of 10. That number is now currently around 20.
While there are unfunded liabilities in the future, the resulting labor demand which will occur when the baby boomers retire will drive salaries sky high (as they did in the late 90s with the dot.com boom). This will increase revenues and thus taxes.
The professor talks about 3 "terrifying" alternatives. The 4th alternative is growth which will lower that P/E. The 5th alternative is a combination of the first 4 which will be difficult, but not terrifying. Also I suspect what we have promised in benefits is a smaller part of our workers' future earnings than many western european countries.
Also, the U.S. owns a lot of valuable assets, and nobody ever counts them when discussing our total debt.
For example, we could sell off the rights to ANWR for a few billion dollars. I bet we could get billions for some of the so-called "parks" which have no value as parks.