Three or four years after the last shutdown (twenty days long!) we had a budget surplus and the lowest unemployment in decades. I'm not saying those things are related, but your pessimism is probably excessive.
That happened during a very strong economy. The economy now is much more vulnerable. An extended shutdown could tip the US back into recession, partly through primary effects (that is, reduced economic activity from everybody the government pays, and everybody they pay) and partly through secondary ones (e.g., consumers putting off spending and businesses deferring investment until things are more clear).
Also, the threats to force a debt default along with the shutdown are dumbfounding. That would, as The Economist writes, "unleash global financial chaos". Poorly handled, it could be worse than the 2008 crisis, especially given what poor shape Europe is in right now.
