These days, when people try to think about alternative currencies, they often automatically think about cryptocurrencies. But before that, other ideas were invented and tried; some of them similar to what you want.
I am pretty sure the idea of automatically decaying currency is there somewhere. Actually, the governments would probably make something like that mandatory, to discourage people from using the alternative currency too much, and protect the position of the official money as something that (allegedly) decays more slowly. Because governments typically want to keep the monopoly on currency, but some of them are willing to allow alternatives, as long as those are somehow “crippled”. (Technically, food stamps are already an “alternative currency”.)
These days, when people try to think about alternative currencies, they often automatically think about cryptocurrencies. But before that, other ideas were invented and tried; some of them similar to what you want.
I am pretty sure the idea of automatically decaying currency is there somewhere. Actually, the governments would probably make something like that mandatory, to discourage people from using the alternative currency too much, and protect the position of the official money as something that (allegedly) decays more slowly. Because governments typically want to keep the monopoly on currency, but some of them are willing to allow alternatives, as long as those are somehow “crippled”. (Technically, food stamps are already an “alternative currency”.)