"[Signal is] really an engine for revealing people's true preferences for messaging, which, for many people, tend to be that they want all the ergonomics of Slack a lot more than they want cryptographically sound secure messaging."
This comparison to Slack makes no sense - Signal replaces texts and makes them end-to-end encrypted. It's a straight upgrade to texting (except, apparently, on iphone, where apple won't let the app send plain old texts and the "drop-in replacement" quality is neutered). It requires a phone number to use, and is linked to that phone number.
Signal is right to be what it is, and if Apple got out of the way, I would insist on replacing all texts with Signal. Replacing my Slacks with Signal or my Signal messages with Slack fails to type-check.