> I can not realistically picture them ever paying the debt off or going back to a zero deficit

"Back" was a long, long time ago. Nevertheless it is probable the deficit will be subsumed by growth nin the immediate future.

> . I bet the most likely endgame will be some kind of future war against The Bad Guys (which they'll make sure to do sufficient propaganda demonization against for the low-brow general public) and then they could use that situation to justify "retiring" (not honoring) the debt.

There doesn't need to be any "endgame" or debt retirement: this is a fundamental misunderstanding. The government is continually paying off old debt and issuing new debt as the US economy grows and government revenues increase. There is no big pile of debt which is continually accumulating and never paid off: the debt is paid off every single time a bond is redeemed. (Nevermind the fact that inflation reduces the amount of debt in the first place! The national debt as a percentage of GDP exceeded its modern numbers in 1945, but it was inflated away.) People often speak of the debt being "called in": this fundamental truth also happens to explain why that can't happen. A bond comes due when it is due, not when you want your money back.

The situation is, indeed, entirely sustainable, unless you disbelieve the evidence of history and mainstream economics. Furthermore, the US government actually owes much of its debt it itself. The government owes itself 5 trillion dollars. This is pretty much just a silly accounting trick: the government could poof all that debt away and nobody would be the wiser so long as the government still funded the programs (such as Social Security) that bought those bonds. And since the government is the sole producer of US dollars, it should not find that too difficult.

There's also the present reality that not borrowing money at the present interest rates would be insane. Real interest rates on federal debt are near-zero and have actually gone negative at some points. People are literally paying the government to hold their money. [1]

You can go even more extreme and consider MMT. There is logic to it, though I suspect human psychology and the general terror of fiat currencies (especially of treating fiat currencies as if they were truly fiat currencies) would hamper any baldfaced implementation of MMT policies. Essentially, fiat currencies can be viewed as being created via government spending and destroyed via government taxation. The only worry the government need consider is the possibility of inflation, which is not (this is predicted by economics and demonstrated repeatedly, including by present experience, in which the Fed has increased the monetary base by 5x[3] while seeing inflation tick between 0-2%[4]) directly correlated only to the money supply and is relatively easily controlled. Governments issuing their own sovereign fiat currencies are working in this system whether they realize it or not.

tl;dr There is nothing to worry about, people don't understand how debt owed in a country's sovereign currency works; there will never be any "reset excuse" necessary unless a future Congress goes absolutely mad and decides to accumulate truly absurd levels of debt (100x GDP) during economic booms or something.

Sidenote: Government bonds provide a secondary, extremely valuable service. They are a safe, stable place to park your money. If the government stopped issuing bonds and only paid them off, there would be a tremendous outcry and people would start either keeping the money in bank accounts or under their mattress (where it does little good) or investing it into the stock market (riskier and probably increases the chances of a bubble.)

References:
[1] http://www.treasury.gov/resource-center/data-chart-center/in...

[2] Debt owed to foreign countries, by amount. Japan will soon overtake China:
http://www.treasury.gov/ticdata/Publish/mfh.txt

[3] http://research.stlouisfed.org/fred2/series/BASE

[4] http://www.usinflationcalculator.com/inflation/current-infla...