Gold and the US dollar, to some extent, fit that characterization as well.

Sorry, I'm not going to be dragged into a debate about if/when the collapse will happen. But if it does, your claim bitcoin is a pyramid will look silly, since it will be one of several assets that will be rushed into as a safe haven from inflationary forces. If you want to take the position such a collapse would cause a flight from crypto assets, be my guest.

The value of cryptocurrency depends on how much people are willing to provide goods and services in exchange for cryptocurrency. If people are just trading crypto:dollar then a dollar collapse could destroy crypto too.

It the dollar collapsed, but the internet & support infrastructure remained, then people might be more willing to accept bitcoin over dollars.

In the mad max future you describe the only currency will be guns and ammo. Crypto requires a functional power grid and internet.

False - there are plenty of scenarios where you have a power grid and ability to reach the Internet and still need to transact illegally to survive.