I disagree.
Bitcoin is literally a proof-of-work in conjunction with a point on an adoption curve.
The proof-of-work part is what makes it tangible IRL, the point in the adoption curve is what makes it relevant to society. Yes, the same thing in a vacuum wouldn't be of much value, but value is a social construct and Bitcoin has that.
I set out to delineate the difference between Bitcoin and the stock market, not the value of Bitcoin. But I'll follow your tangent.
"Proof of work" is not proof of useful work. It's proof that an amazing amounts of electricity was wasted on what amounts to little more than digital thumb twiddling. Useful to ASIC manufacturers? Sure. Useful to mining contract resellers? Yeah. But useful to society as a whole? Not really.
How useful is a currency that consumes 10% of its market cap, each year, to secure? Well, if Satoshi is right, and price of mining does approach the price of mined coins, then Bitcoin is costing just that: ~10% of all the coins in existence were mined this year
It is arguably the only way to create an immutable, decentralized ledger. Not useful work? Very much useful since without the work the chain is not secure.
How useful is a currency that is easily attacked and vulnerable to corruption?
You are starting with a few dubious assumptions:
1. An immutable, decentralized ledger is the right way to do money.
2. Ten percent, yearly, of the entire worth of Bitcoin (or any currency) is a reasonable amount to spend on security.
3. The current supermine distribution is decentralized and immune to corruption.