> For crypto to have value, someone else needs to pay Dollar/Euro/whatever at an exchange. That's the value of crypto - the exchange rate.

This is true for every economic good, including paintings. Whether the value is "inherent" or not is irrelevant, you need a transaction or the value is just theory. Culture could also forget about the significance of the Mona Lisa, it has happened before, see for example https://en.wikipedia.org/wiki/Archimedes_Palimpsest, a mathematical text that our culture considers having very high value, but was overwritten into a prayer book by a culture that considered the work less valuable than the material it was written on.