You could fund the first 5.5 years of your retirement with govt. bonds, but if the bulk of your retirement funds are in a Roth, you're not tied to the paltry government bond yields. You're 5.5% example above barely outpaces inflation in many years.

You can also afford to have $10-20K of taxable income every year. There's a threshold around the poverty level that is barely taxed, if taxed at all. That's why I won't put all of mine in a Roth.