[TheNewsIsHere]

This analogy isn’t convincing for me. My letter box and my email inbox both have a finite amount of space, but I can (effectively) infinitely extend my email inbox either “online” or by archiving it someplace.

I cannot achieve this with my letter box.

That assumes large volumes of important mail. Even without that though, an email inbox is a natural and “first class” place to store an email, whereas a letter box is not designed to be effective at any sort of reliable storage.