You've missed the point. What if you need to work on the circuit connected to the other side? Suppose an outbuilding is wired on a separate 1P+N breaker and you need to work on a circuit in there? No need to turn off the whole house, just isolate that circuit. You've saved money by having the breaker and separate isolator.
I have not missed the point, you simply see a huge inconvenience where most people do not.
Did you know that connecting two relay contacts in series can double the voltage rating because it quenches the arc more effectively?
And under which standard is this tested and acceptable? Again, the device is designed to be capable of clearing any fault it should reasonably expect to encounter in service.
Just because you cannot see it, it doesn't mean it isn't so. Reread the thread.
I have repeatedly asked for you to actually detail the scenario, instead of just repeating "broken neutral" like a mantra.
I shot down the theory of a high fault current through neutral shorted to earth, as a 1P+N MCB cannot clear it, and likewise it cannot be relied upon to protect from contact with broken neutral at high potential because
this is not what MCBs do, RCDs are the protective device for human contact, as well as neutral to earth fault currents.