OK, "live touches frame" is a fault - it can happen and that's why we design in features to deal with it.
"Current flows from metal frame to neutral" is a different issue.
Here in the UK, that would need a separate fault as the frame(case) would be earthed. So we've be looking at two faults for that to occur - and the device would have stopped working with either one of them. Otherwise it would need a fairly improbably fault - e.g. power cord pulled such that all 3 cores get disconnected, L&N both tough the frame together, but E does not touch the frame.
Yes, agreed in terms of earthing. I do understand and appreciate why earthing is critical for safety.
My scenario is purposefully leaving out any earthing of the frame as I want to understand how an RCD will behave (even if such a setup is highly unlikely and not advised).