But:

- It exists.

- It's ... kind of ... not owned. (Though as TFA notes, this is only vanishingly true any more.) But compared to most alternatives, it is less owned by any one entity.

- It's supported over a wide range of devices, from smartphones to mainframes.

- It has extensive mindshare, use-culture, associated processes, etc.

- It is largely agnostic as to content. Yes, plain text is most native, but through the majick of MIME, et al, it'll manage just about anything.

If you're going to call for email's end, you'd best have a highly viable alternative or replacement in mind.