Fiat currency savings accounts almost universally have negative rates of real return. (The small amount of nominal interest is cancelled out by a couple percent per year of inflation).

Sure, but you get the other advantages of bitcoin. I interpreted the question as coming from a bitcoin user: "why use bitcoin+demurrage if I can just use bitcoin?"

If you have already decided that a savings account with interest is an even better way to store value, then my argument indeed doesn't work.