Bitcoin is still more expensive than the banking system among first-world countries. How much can it be worth to send money to third-world countries of no banks are willing to set up there?

Bitcoin BTC is currently more expensive than the banking system in first world countries because of the specific technical choices they have made. When I first used bitcoin it was free to the user (i.e. it was paid for by issuance). Other cryptocurrency systems that have made different technical choices are much cheaper these days - bitcoin BTC is a bit of an outlier in that regard. I personally have been experimenting with USDC on Argent wallet (a zksync L2 on ethereum). It's easy to use, quick and the transaction fees are around 20c at the moment (and likely to reduce further), and I'm sending a stable coin that tracks dollar.

You could always use the crypto currency as the store of value there, just as people did with MPesa. You don't necessarily ever have to convert it to a local currency.

MPesa doesn't store your money in some kind of private currency with a fluctuating value. If you're thinking of it as storing "minutes" as some American cellphone providers would, that's not right.

You deposit and withdraw Kenyan Shillings, or your local equivalent, just as if it were a shilling-denominated bank account.

Yes, and you could make this hypothetical Kenyan cryptocurrency work the same way. One KCoin is one shilling, redeemable for minutes.

where are you getting these $1 international wire transfers? Sign me up!

SEPA transfers cost €0, which (still) is less than $1.

Ok, let me send dollars from the US to Japan via SEPA real quick.

You asked “where”, and the answer is “in the SEPA countries”, which at least includes most of Europe.

Then I apologize for my imprecision! I meant "what financial institution will provide me and my counterparty this service for the currency we currently use in our current countries of residence" rather than "what pair of countries can me and my counterparty relocate to to obtain this service in at least one currency."