I guess I'd need an exact definition of "money". Is it just paper money, savings accounts... is it sovereign debt? Thats what the backing of "money market" accounts is btw. If it included sovereign debt just expiring that would cause everyone holding treasuries to replace it with other assets, ie gold, commodities, or real estate. That'd cause inflation like the USA has never experienced before. So, yeah it would put lots of people into poverty.
Apparently the proposal in the OP is just for fiat currency specifically. I'm including "money in the bank" as part of this.
But I think you have a good point. The whole thing gets really bizarre really fast, if you start to think about it.
I have a line of credit with a credit card company. I use the card for pretty much every transaction I make, because it gives me airline miles with my preferred airline.
Is that line of credit money? Would I need to pay interest on it, or see it dwindle, month by month? If I set up an automatic pipeline, from my payroll direct deposit to my checking account to my credit card bill, is there ever any money hanging around in my account long enough to expire?
What about my retirement funds? I'm not saving a lot of money in a savings account. Rather, I put it into diversified investment portfolios and whole life insurance policies. When I need to extract money from those things, I'll take what I need, and spend it immediately on the things I need it for. Would I need to pay interest on the nominal dollar value of my 401(k)? Would I need to pay interest on the current buying price of the company stock I'm acquiring through my employee stock purchase program?
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Would I need to pay interest on the land I invest in?
Actually, that happens already. It's not called interest, it's a property tax. But the principle is broadly similar: You can't just sit on it, you have to use it. Either spend it or invest it. Or lose money on it.
So maybe "tax" is a better word than "interest" in this case. You pull out a ten dollar bill at the cash register, but it's two years old. So now, in addition to the sales tax, a separate "freshening" tax is assessed, based on how old the bill is when you tender it as payment.
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I keep thinking about the idea, but it keeps not seeming any less crackpot.