I would say that having a private intranet only NNTP server with discussion groups would be far better than just using email.
For one thing, you already have grouping of general topics on a server level rather than having to take the time to create rules/filters and IMAP folders in using the email client. Second, it's much easier to access previous messages by just subscribing to the group (rather than having someone forward you an email which you have to read from bottom to top to get a summary of what was discussed). Lastly, it separates discussions from personal communication (emails sent directly to you).
IME, it's no different than using email other than the fact that you'll have a Newsgroups field instead of a To field. Composing and replying are pretty much the same as email. And each subscribed newsgroup could just as well be thought of as an email folder.
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> ... everyone who wants to replace email ...
I like email too. In the office, though, I've found it best used as one tool among many. In my last job we used email, IRC, a bug tracker, etherpad, pastebin, mediawiki, git+gitlab, and another "fulltext search most of the above" service.
I don't know if that's too many things -- are there good usability wins from combining a few of these things into one? Maybe not -- so long as everything we want to "point to" and share has a URL, maybe we were served just fine with that many separate things.
Still, it's easy to see why someone might say "Look how limited email is, it's not suitable for all of these extra tasks. I'm going to make something much more versatile."
This is something I wish was a standard part of email, actually. There are times when I want to refer to an email thread (say, in a ticket in our issue tracker) and I have to resort to either quoting all the relevant parts verbatim or saying something like "for context, search for the email sent on 2015-11-20 by foo@bar with the subject line 'widget is broken'". I'd love something like `mailref:[identifier]` to be widely understood by browsers and email clients.
Mailing lists that are readable on the web partially solve this, but it's still not great UX to be sent away from my email client just to read another email.
Email is still the most lucrative marketing channel you can devise. I've yet to find a better intra-office communication tool. If I'm having trouble doing it with email, I immediately switch gears and either get up and go talk to somebody or to go do some fact-finding.
Email allowed me to maintain long-distance friendships where otherwise we would have drifted off. Email is easily categorizable and sortable. My workflow takes close to a minute to clear everything out to where only open loops are in my inbox.
I look at everyone who wants to replace email with someone else like I would a 25 year old bro who thinks he's going to 'disrupt' something ridiculous like politics. Harmlessly stupid at best, dangerously power-hungry at worst. Look somewhere else to build your empire, buffoon.