Ownership of a stock is a claim on the net value and future profit of the company. In a theoretically efficient market with perfect information, stock price would be fixed; its value would be the same as the net present value of all income streams associated with that stock. But we don't have perfect information, especially about the future; so a certain amount of the stock price is formed from expectations about future profit.
It is of course possible that there can be a bubble of expectations in a kind of echo chamber that cause a stock to act like a Ponzi scheme, but ultimately over time there must be returns. The return can be as a stock price rise (but that can't go on forever) or dividends; if the cost[1] of the money invested in the stock doesn't match the return over time, then people will sell the stock and it will fall in price. If too much of the stock price is formed from aspirational expectations of the future (i.e. a bubble), then the price fall may be drastic.
[1] Money has a cost; for example, compared with the interest rate on a risk-free government bond. If an investment isn't making at least that much, it's losing money.
In principle, you're right, currency is just traded like anything else; but the fundamentals are different. A company is (presumably) engaged in profitable production (the future profit, i.e. it does something) and is composed of various capital assets (the net value, i.e. it has something). A currency has neither attributes; its claim on future production and assets is only valid in so far as people have confidence in it and will take it in exchange for other things. A share, meanwhile, is a direct claim on things themselves.
Currency can have its own features that make it worthwhile. No matter what happens with btc's popularity, as long as I have one other party to trade with, some features will be available to me through btc that are not available through normally denominated national currencies. Can't I buy btc because I think these are valuable features and their desirability will drive interest and users, just like I would buy Google because the desirability of its features drives interest and users?
A share of an ETF is a claim on a financial asset that is backed by claims in other financial assets, usually stocks. So while it is one level higher than directly owning securities, it is still a claim on productive economic assets.
It is of course possible that there can be a bubble of expectations in a kind of echo chamber that cause a stock to act like a Ponzi scheme, but ultimately over time there must be returns. The return can be as a stock price rise (but that can't go on forever) or dividends; if the cost[1] of the money invested in the stock doesn't match the return over time, then people will sell the stock and it will fall in price. If too much of the stock price is formed from aspirational expectations of the future (i.e. a bubble), then the price fall may be drastic.
[1] Money has a cost; for example, compared with the interest rate on a risk-free government bond. If an investment isn't making at least that much, it's losing money.