Sure, I was only making an analogy -- people refinancing over and over, vaguely like governments pay T-bill interest by issuing more T-bills, where there's no expectation that the principal can ever really get paid back.

I was about to mention Fannie and Freddie as a footnote about how the government might be involved in all this, but didn't think it was a big deal. Yet Google tells me they end up buying 70% of mortgages. So there's some flow here that I'd do well to understand.