"By that definition the USD is a ponzi. No, national currencies too fail to fit the definition, because people do not "invest" in them with the expectation of gain" <-- The 6.6trillion/day forex market begs to differ.
Well, insurance industry is a negative sum game (zero sum minus costs of running it) in which all the buyers expect to lose money and all the sellers make it. Still, it's a good idea to buy insurance in some circumstances as utility of money curve is different different for the insurance buyer and insurance company. Such transactions are good for both sides (as they maximize their utility even though the buyer losses in pure money terms).
Now apply that to Forex. There are players who needs to hedge against target currency devaluing and others who are willing to take the risk premium. That you can gamble on it if another matter but then again, you can gamble on many different otherwise useful financial instruments.
Nobody "invest" in USD. You invest in assets which you expect to appreciate. People investing in Bitcoin do so expecting to control a portion of a finite supply, which defeats its purpose of liquidity.
People absolutely invest in USD, in expectation of appreciation. South Korean banks sell saving accounts denominated in USD as an investment product. Money flows into it when USD appreciates and flows out when it depreciates.
Also it is not true that "governments make their currencies slightly inflationary precisely to discourage hoarding." In fact, this is done so that there can be economic expansion. It is true that some economists have advocated for deliberately trying to increase inflation to spur spending, but it turns out that actually forcing inflation is not easy (see also: Japan for the past two decades).
Gross daily trading volumes are a poor gauge of how much capital investors have deployed in a particular market, seeking returns. A single international transaction may induce several forex trades, such as when the sale price of goods is denominated in USD or some other non-local currency. Combine that with the complex, usually leveraged hedging strategies employed by banks and multinational firms, and it’s easy to see whence such a massive figure comes.
There are a lot of people who are "investing" in Bitcoin right now not out of expectation of real gains, but as a way to maintain assets against an inflating dollar... nominal gains, but not real ones.
Haha, yes if you're just a "weekly" investor. But for me who invested years ago, my assets begs to differ. Back then there were people I know personally that like you who also said that it's a bad idea to invest in crypto, I'm very glad I didn't listen to them haha. Now I just tell them it is not yet too late before global adoption.
Having said that, speculating on FX is a zero-sum game, which I'm not generally in favor of.