> there should be at least ten alternatives in any diversified set of critical infrastructure service providers, all of them instantly replaceable / forced to provide interoperability...

And does anyone actually know how to actually implement this, at the scale required (dealing with billions of transactions daily) in a way that would resolve the problems we are seeing?

It very much seems like a data access problem; places can't access/modify data. The physical disks themselves are most likely fine, but the 'interfaces' are having troubles (assuming that the data isn't stored on the devices having the issue).

But in any case how do you design a system that, if the 'main' interface is troubled, you can switch over, instantly, seemlessly, duplicating access controls, permissions, data validation, logic etc.

There is a reason everything is centralised because it makes no financial sense to duplicate for an extremely unlikely and rare chance. The world is random and these things will happen, but a global outage on this type of scale is not a daily occurance.

We'll look back in a few years and think "those were a crazy few hours" and move on...