I'm saying there are no situations in which I'd want more than a single physical phone. I've used more than a single physical phone, like when I'm outside the US and want a local cellphone number but don't want to lose my US number, but in every situation I'd prefer to keep a single physical phone.

You've never lived or worked in a building with terrible cell phone reception

Switch providers. Though, ideally, I should be able to get calls through VoIP on my WiFi.

had a dead battery but still really needed people to be able to get a hold of you

It's easier to find/carry a charger than it is to notify everyone that needs to be able to get a hold of me of the new number.

had your phone stolen, or had your simcard go bad?

These are exceptional situations. I can get a phone replaced within a day in such a scenario. If it's urgent I can borrow a friend's cellphone.