There's a point there. In terms of getting text from one device to another so our friends know which bar to meet us in, email has almost certainly been replaced by more streamlined services where we only have conversations with our friends, without the clutter of "summer bargains" and "TV Sale ends tonight."
However, here's the real point that email doubters needs to be asking themselves. How did people sign up for these services in the first place? Sure, it may have been a Facebook log-in, but then you could ask the same question about what underpins that original social account? The answer will be the user's email address.
Now, I dispute the "email needs reinventing" argument that suggests Millennials are switching off from the channel. I bet the average Millennials's inbox has as many newsletters and branded messages as the average person from an older demographic. Even if it is a little lower, however, the other methods brands will be utilising to get targeted messages to that consumer will revolve around email.
Whenever a person is surfing the Web, you can almost automatically assume their email address is identifying them to advertisers. The identify should be hidden from the final user of the data but the fact they are 25, like playing tennis and they've been researching new work shoes will be laid bare thanks to email identification. Plus when that person takes online research in-store, it's likely to be an email based loyalty programme or an iBeacon that allows the gulf between the channels to be spanned.
So there may well be a need for inboxes to have the most apt messages prioritised -- this is the typical argument of the "email is dead" brigade. But these things have a way of sorting themselves out. Stand at any bus stop and you'll see people of all ages open their inbox, select a batch of messages and delete them. We've probably also all got into the habit of filtering offers and coupons to a particular folder or even having a separate email address for newsletters and brand communications.
People will evolve how they use email for the obvious reason you can feel swamped otherwise. However, the legacy argument misses the main point. What other means are marketers going to use to strike up a conversation with a prospect that can keep them loyal and hopefully upsell to later on? Which channel also allows you to track their behaviour to better inform wider digital campaigns and how else are you going to track behaviour in the mobile world, which has no cookies, to activity in-store?
If this sounds like a troubled channel in need of reinvention, I'll wager many other disciplines would happily swap places.