I wouldn’t call myself a cryptocurrency acolyte but I think this is a substantial oversimplification, and your “theory” is not the ethos or point of increasing difficulty at all. The diamond analogy is flawed - they absolutely have no intrinsic value until they have an application; therein is the value.
Bitcoin’s primary usage [at this time] is for transferring money relatively anonymously. As soon as a sum goes through two or three transfers it becomes basically impossible to trace. That application means it has a value to a certain group of people. As soon as that can be policed, sure, the value will likely drop, but the very point of many crypto currencies is a self regulation component that cannot be altered by outside forces. This is how the concept of Bitcoin came to exist and why statements about it no longer existing demonstrate a fundamental misunderstanding about its purpose or function.
Bitcoin was just the first, and is extremely inefficient. It was an exploratory application of blockchain technology that was latched onto because of the massive value in the aforementioned application.
There are many other cryptocurrencies that have very practical uses with far lower computing requirements - Ethereum 2.0 is going a long way to solving these embryonic issues of a new technology. Blockchain technology is here to stay, cryptocurrency by extension will be too.
I repeat, I am not a cryptocurrency acolyte. I just think it’s important that commentary on such a complex topic is at least relatively accurate.