You hit at the crux of the issue here...and I wish I'd said this with the post you quoted.I think the critical difference is that with a mortgage, principal gets paid down every month. With government, the best you can hope for is a balanced budget in the best of times, and of course, more deficits most other years. Its kind of like those mortgages where you only pay the interest and not the principal.
People age, populations don't. Mort-gage is literally "death paper" (read: contract or agreement). There's profound implications for that when the population in aggregate is continuously in their earning prime.
There's of course a very instructive examples of an aging population (Japan), but the manifest differences between there and the US makes our debt burden sustainable. IMHO.
