On the other hand, they’re legally (often constitutionally) protected in most countries, so that you have some assurance that no private person will steal or read your mail between the moment it’s dropped in a public box and the time you pick it up from your own mailbox. That makes them suitable for sending cheques, tax forms, initial passwords for online resources, some legal notices, etc. The current email or direct-message systems don’t have these legal protections. Once we’re only communicating electronically, there will be very little to stop governments from reading content or even censoring it.

That’s the real meat of the issue. Right now, the decentralized and relatively insecure way that emails are sent and routed means that there’s always the possibility that it can be tampered with, read, blocked, etc. without the rather extreme difficulty that it takes to do so with physical mail. That’s why official correspondence is pretty much either sent via physical mail or set up with one of those “Message Center” type functionalities where you log into the corresponder’s site and read the message there without it actually being sent.

That said, I can’t remember the last time I sent a letter that wasn’t compelled by some organization who required a physical letter. Most everything is emails, texts, or those message-center type schemes.

I think at some point, someone’s going to integrate some sort of digital signature functionality and secure messaging into smartphones as a default, and that’ll start to chip away at a lot of the remaining physical messaging. But until that’s something that you just set up automatically as a mandatory part of setting up the phone/account, we’ll still require physical mail because there are a lot of people who are ignorant of the security aspects, or who are just lazy and won’t bother setting it up.