Bitcoin holdings don't pay interest either and its purchasing power swings wildly. If you want to guard against inflation in the long run you have to own productive assets or lend your money to someone who uses it productively.
What if next year some other cool new "currency" comes along and no one wants your stinkin' bitcoins any longer? Does it matter that supply has a cap? No. It matters as little as the limited supply of paintings created by some forever unknown artist.