I don't think most exchanges are rooted in scams no - I'd say they're rooted in a greater fool scheme, given that the main purpose of buying crypto is to sell it for profit and it isn't really used for exchange outside a few niches like darknet markets.
Then I think you are assuming your premise/begging the question. You constructed the conditions in a way that leaves no room for a valid example. If you think crypto itself is a greater fool scheme then purely definitionally there can be no example of a non-greater-fool-rooted crypto business.
This is what I've observed, it's not a prima facie assumption. I was initially optimistic when bitcoin came about. Most of the industry seems to be based on speculation and selling "x but on the blockchain" solutions to nonexistent problems. I also know people who are trying to use blockchain tech to solve real problems, but they're in the minority.
