It's fine as long as t-bill rates match or exceed inflation. Then you can avoid losing purchasing power by just putting your money in the world's safest investment. Over the past century, t-bill returns have slightly exceeded inflation on average, though there have been periods when they didn't.
Stash paper cash in your safe and sure, you lose purchasing power. Use fiat money the way it's designed to be used, instead of using it like gold coins, and it works better.
Us debt as a fraction of GDP has doubled this century and roughly quadrupled in my lifetime. It would seem to me that eventually t-bills will not be safe.