We're in agreement that lying about backing is wrong in any case, no argument there.

I in particular object to the notion that things of value should be 100% backed by actual USD, in bank accounts or cash. That's not how liquidity works, yet a lot of people intuitively believe this to be so.

They said it wasn't hard, not that it was necessary in all circumstances.

Banks and gold deposits re-lend money because it makes them money and they can sell the storage service for cheaper. If you want something that won't be loaned back out then you can buy that service pretty easily.

And stock prices changing is a very different thing. The actual supply is there, and entire markets caps can and do get sold in things like mergers.

The supply of a stock in itself is meaningless, the only point of a stock is its USD value representation. In the case of a mass sell-off, sure, the same amount of stocks exist, but its monetary value sinks. Hence the market cap goes up in smoke.

This is also why a fraction of a market cap (say a few billion) can dramatically move a stock price of a 1 trillion market cap.

Same for Bitcoin. If its market price crashes, the same amount of Bitcoin exists. Yet this means nothing as only the BTCUSD pair is worth anything.