I still use E-mail, but only for reading, not writing. Hence, this message is in this channel but not as a reply to your E-mail. So, what has become of E-mail is similar to what happened with Wikipedia.
Like Wikipedia, E-mail is over-regulated, hence people avoid using it (for writing), but Slack is now getting over-regulated too, so soon we’ll have to migrate yet again to the next ~big~ new thing..
The iron law of oligarchy is as play here, in regard to who can write communications (but a small few) versus who can read them (everyone).
About permissions. The “tried-and-true” standard that only a few have write access, and all other writes have to go through those few people. Unfortunately, this creates a communication bottleneck, and the result is that most people simply decide not to bother trying to communicate due to the overhead. When communications cannot happen quickly, most people will decide to stop trying, rather than wait out a full review process, especially if the point is to get work done that is not solely centered on communications.
Ultimately, this means that the most useful communications media is that where people can get their messages out as quickly as possible. Another factor of a useful communications medium is how many people use it to write. If it is only being used by a few to write, surely it must not be very successful.