> A similar question can be asked about USD: why would you ever save it instead of spending it immediately, since it's only going to lose value over time due to inflation?
Except for relatively small amounts for liquidity for known or potential immediate needs, it doesn't make sense to hold US dollars (or any other modern fiat currency), it makes a lot more sense to invest them in productive assets which can be converted easily back to US dollars (or other fiat currency) when necessary.
This is not an accidental feature of modern fiat currency, its pretty much the whole point of the transition from commodity-based (e.g., gold standard) to fiat currency -- so that broad downturns don't have a dangerous positive feedback effect where poor investment market conditions lead to currency being a more attractive place to hold money than the investment markets, which leads to even poorer performance in the investment markets and even more people pulling money out of those markets and into currency, etc.
> But clearly, if bitcoin ever becomes a major currency and the deflation levels off [...]
Please explain how increasing interest in using bitcoin is going to cause the price increase relative to other currencies or goods to slow down.
> [...] there will be plenty of motivation to spend bitcoins, the same way there is currently plenty of motivation to save USD.
Seriously? How about buying a house? Buying a car? Going to college? Going on vacation? ... Most people cannot save up enough for these things overnight, and they aren't going to put their downpayment savings fund into stocks because it's not worth the risk of losing it for them.
Um, you know there do exist lower-risk investment vehicles than stocks, right? Back when there was inflation, you'd be a fool not to put the $10,000 you're saving for a house into a 1- 2- or 5-year CD. And investing in I-bonds for their kids' education is still something parents do.
> Seriously? How about buying a house? Buying a car? Going to college? Going on vacation? ...
In all those cases, saving USD as dollars is generally a suboptimal idea (unless its over a particularly short term, but that gets back to the short-term liquidity, not long-term investment, case.)
Using the dollars to buy investments that are expected to be worth more when converted back to cash is what you usually want to do.
That's a fine idea, but few people will do that in practice because they don't want any risk at all in losing the money they're saving for those things. People generally invest for the very long-term, like retirement, not for things like house downpayments.
Consider the humble savings account. Those accounts may be dollar-denominated, but under the hood they're really a type of security. You're making a loan to the bank at interest - a special kind of loan that you can call at any time, in part or in full, but a loan all the same. The bank then takes that money and loans it out at interest to people who want to borrow money to buy houses cars, whatever.
There is risk involved - the people at the end of the chain might default on their loans, in which case the bank loses their money. Historically, if the bank lost enough money then they wouldn't be able to pay back the money they borrowed from you, in which case you also lose it. Nowadays banks are required to carry deposit insurance, so instead when the bank defaults the FDIC loses their money, but you still get paid. But if enough banks default then the FDIC also won't be able to make good on their guarantee to you, and you're still out your money. Meaning that when you stick your cash in a savings account, it's not really money anymore. It's an IOU - a promise to give you money when you ask for it. Probably.
> few people will do that in practice because they don't want any risk at all in losing the money they're saving for those things.
Many people do it in practice. Stuffing dollars in your mattress is "saving dollars". Loaning money at interest who will aggregate it with other money and invest in productive assets is an indirect investment in productive assets other than dollars.
Stocks aren't the only investment option. Safer options exist, e.g. corporate bonds, money market funds, and certificates of deposit. All of those are a more productive use of capital than socking it away under a mattress or in a Bitcoin wallet.
Except for relatively small amounts for liquidity for known or potential immediate needs, it doesn't make sense to hold US dollars (or any other modern fiat currency), it makes a lot more sense to invest them in productive assets which can be converted easily back to US dollars (or other fiat currency) when necessary.
This is not an accidental feature of modern fiat currency, its pretty much the whole point of the transition from commodity-based (e.g., gold standard) to fiat currency -- so that broad downturns don't have a dangerous positive feedback effect where poor investment market conditions lead to currency being a more attractive place to hold money than the investment markets, which leads to even poorer performance in the investment markets and even more people pulling money out of those markets and into currency, etc.
> But clearly, if bitcoin ever becomes a major currency and the deflation levels off [...]
Please explain how increasing interest in using bitcoin is going to cause the price increase relative to other currencies or goods to slow down.
> [...] there will be plenty of motivation to spend bitcoins, the same way there is currently plenty of motivation to save USD.
What motivation to save USD?