One possible way to deal with this and very nearly return to the former freedom-to-tinker status quo is to send the bank your custom OS along with a computer-checkable formal proof that the bank's app, while running on your OS, behaves as it would be expected to under the stock OS. With homomorphic encryption, it might be possible to do this without revealing your custom OS, only its one-way hash. The bank can then verify that the proof is correct and then accept transactions with attestation from your custom OS. This would enable installing a custom ROM that can be used for online banking without having to go through some cabal/consortium. The only caveat is something of this magnitude has never been done before. It's a research project for sure. It would take many man- and compute-hours. But it would be very cool.