The alternative is to let your money assuredly decay in a savings account that won't exceed the rate of inflation.
Retirement savings are meant to be held for the long haul. The market will experience lots of fluctuations in between now and the time you'd be making withdrawals, but statistically speaking, if you diversified properly, your return over 30+ years would almost surely come out positive.
If you're really worried about the possibility of default, ride out the next few months until either:
1. Our idiot politicians finally stop going full-retard or...
2. Our idiot politicians explode the economy, in which case you'd buy up index funds when they're cheap and the recession begins to turn back around.
In any case, investing for your retirement isn't really an option; you HAVE to do it, and not using a 401(k) that's available to you, especially if your employer matches your contributions, is a big step in the wrong direction.