One extreme: At some point far in the future, perhaps dollars aren't a thing anymore - the last people holding dollars are the greater fool. (Note I still hold dollars and ETFs and conventionally safe investments, that's not a knock on the dollar.) But over a long enough timespan, there's a greater fool. There are just a lot of people along the way who got a lot of value out of dollars.
Another extreme: There are things like beanie babies where there is a very short amount of time to find a buyer, and the last one to buy the beanie baby before the fad stops is the greater fool. This is over a much shorter timescale, and there are only a few people who get a lot of value out of beanie babies.
Crypto is somewhere in between. I think the black market use case means crypto is closer to the dollar side of the spectrum than the beanie baby side of the spectrum. I think there's a decent chance there is always a greater fool in decades, well long enough for me to get out while I've gotten value out of it.