Most orgs still rely heavily on email, and most emails should be more than two lines to be useful. If it's only two lines, Slack it.
big tech CEOs and major politicians use email almost as if it's an instant messenger: short, informal, to the point.
I guess it kinda bites them in the ass because the way I know this is via email leaks or subpoenas...
But it does mean that people can use whatever clients they like, and they can be across multiple organisations without weirdly expensive plans and administrators bridging instances or what have you.