Once we hit 24 trillion, many think we'll be toast. Obama will probably get us there. He just may triple the debt he came into before he leaves office. This is how every 3rd world country starts out - they print, borrow, and spend their way there. The US govt was intended to provide protection for citizens from fraud and harm - military, borders, etc. Not to provide a middle class cradle to grave lifestyle for everyone.
And with the added taxation socialism imposes (that mostly goes into the pockets of the few government hacks in charge), you eventually end up with more or less a dictator in charge (and a few other cronies of his) being rich and everyone else being poor.
The biggest problem here is the fact that we don't tax many items we import like most other countries. Do you think we'd still be building stuff in China if a 20% tax was added to it? The US used to fund it's budget on these taxes but now we just take it from all of the money we earn and large corporations figure out ways to avoid. All the while everyone in the middle gets hit hard in the deal.
You are making the assumption that US dollars will continue to be the default currency of the world. 70+ years ago the British pound was the default currency of the world. China is attempting to knock out the US dollar as the default currency, many oil producing nations will now sign contracts in other things besides US dollars. China is openning development banks, which is a way of slowly knocking out the USA dollar.
Good point. The "default currency" is pretty much decided based on which country is the international economic superpower. Right now it is still the USA but things could be very different in a couple of decades.