You just made the compellling case in your first paragraoh
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That's compelling for us, not for 99% of people. For them, money is a proxy to products/services, not an ideology.
Guarunteed offramps into the real economy are more valuable to them than self custody.
i feel and agree with the spirit of your take, but such a function would be impossible...and in fact to force individual ownership of cryptographic keys would only be possible with a centralized authority in charge to enforce. thus, the only option is for a critical mass of people to actually want to hold their own keys.. and that may not happen.