Earlier quoted context omitted.
I was clearly misunderstanding how TIP yields worked, I'll read more about it. Thanks!
The money illusion got you. With demurrage currencies, everything works like TIPS. Yields may be negative but they are free from inflation and deflation. It's kind of weird that people are choosing the money illusion over a negative yield/interest rate. I would rather have no inflation and see that the yield is negative than unpredictable inflation where the yield could be absolutely anything and I simply won't know.
Not at all true of historical demurrage currencies, with are subject to inflation/deflation as normal but also have reduced nominal value over holding time.
ISTR the term has been resurrected for crypto products, but I can't find any details of how any crypto demurrage currencies work that would confirm or deny that they are somehow insulated from inflation/deflation and only change buying power by the demurrage charge, though I can't see it being likely to work in practice.
In theory, you could have what is basically a “stablecoin” managed against a price index like the CPI-U instead of an actual fiat currency and then add a demurrage charge on top of that (which could provide stabilization resources, so it's not completely irrelevant to function).