Email and instant messaging aren't in competition they fulfill different communication roles and while each can be used as the other (there isn't a deep fundamental difference) each is habitually used in a very different manner. Asynchronous notifications about large tasks shouldn't be done in a medium where that notification is possibly going to scroll off of a screen in a few hours[1] (maybe I'm busy and want to review that long document sometime next week) - while low friction direct communication becomes sloggy through emails - nobody I've ever met would ever email AFK before going off to lunch. And, specifically, I wouldn't want them to mass-email the whole company with ever status change - being able to see availability at a glance makes communication smoother.

Companies should use both.

1. Message reminders in most clients can sorta do this but I still prefer emails.