Very well made post mate, it's getting rarer and rarer for there to be people who actually have seen all/most of the crypto-currencies, and haven't fallen to the marketing jargon and come back with a clear mind.
Medium of exchange(MoE) aside, I cant think of another legitimate use case for decentralisation.
This particular line in your post caught my eye, for a particular reason. There's been a lot of new crypto-currencies that have been made the last couple of years, and a lot of them add random, useless features like "decentralisation" or "blockchains" when they, to simply put it -
don't need it.