> There's a reason many banks still have mainframes powering large portions of their internals.

There are many reasons. Institutional inertia for one, a lack of sufficiently knowledgeable people another. Banks are not innovating because you can't get around them anyway. And personally I don't mind, I'd much rather have my $ parked with an entity that moves slowly and predictably than with some 'move fast and break stuff' outfit that gives me a heartattack 3 times per year by showing me a $0 balance and no support department to call.