> Is the rate of lack of use of bank accounts ever likely to be public knowledge? There's zero incentive on the banks' side to publicise that kind of information.

Public authorities can estimate this (FDIC in the U.S. has done so) and general trends should be evident (if half of previously unbanked Indians get bank accounts we won't need banks to tell us).

But fair - just the rate of Bitcoin wallet creation then? Maybe compare to global/country population growth.