Historical patterns in financial crises, mostly. Over the past couple centuries, once sovereign debt rises over a certain percentage of GDP, countries have invariably defaulted, with the shock coming very suddenly and with a lot of pain and austerity for all involved.
The book is extremely boring, but 'This Time Is Different' has a lot of data on past financial crises, which as far as I can tell is legit and untampered with. Yes, this time could be different, but I don't see any compelling reason why we'll be an exception to the pattern. I'd argue that US sovereign debt is perceived as safe right now only because other countries are further down the same path and because the market's collectively whistling past the graveyard.
Projected spending on Social Security and Medicare is the other reason - these aren't 'unfunded liabilities', as they're frequently described by conservatives, because Congress is under no obligation to maintain Social Security and Medicare at current benefit levels. But if we were to maintain current benefit levels, it's quite a bit of money we've got no solid plans for generating.
I could be wrong, but I suspect something has to give.
Complete agreement on the need for more vocational training.