But they're not adding a "money" transfer option, they're adding a "MobileCoin" transfer option! For the overwhelming part of society, these are not interchange

But they're not adding a "money" transfer option, they're adding a "MobileCoin" transfer option! For the overwhelming part of society, these are not interchangeable terms. When people want to send "money" to other people, they usually imply that they want to send units of the local currency, like USD or EUR. And they usually imply that the value of these units should stay the same during transfer. If I want to pay my share of a restaurant visit to my friend who covered the check, I'd like the 30$ I'm sending to still hold enough value when they arrive in his bank account to actually cover my share. A cryptocurrency intermediate that swings +/- 20% in value within minutes (and that we both have to pay conversion fees in order to acquire/redeem for $) is of exactly no use at all for such a use case.