Disclaimer: I hate threads in slack
When you already have too many channels, and then they give you the ability to have too many threads.
I feel like they do a worse job at the supposed value add of being able to archive and later refer to a topic of conversation than channels simply because they are so light weight and proliferate so many of them, potentially.
Plus it's really jarring to be pulled into a bunch of threads.
Slack segregates by interests and social group, and bundles that with strict gatekeeping. There is no way for people to remix that to suit their own preferences. You can only fragment existing communities more.
I would love a slack multiclient where I can put channels I care about side by side, regardless of origin, and keep the rest out of sight. Instead now everyone has their own #random and #offtopic and so on. Cruising between Slacks and Discords is like navigating a hall of mirrors. If there is disagreement, the only solution is complete schism.
As much as I'd like to live in a world where open protocols and federated services evolve as quickly and turn out as well as proprietary solutions, that's apparently not the world I'm living in. Email, as much as I love it, is basically stagnant. IRC existed long before Slack, and if it had been truly successful, Slack would never have existed because there would not have been a market niche. And having recently written a bunch of XMPP code to talk to my vacuum [1], I was entirely underwhelmed. Whereas Slack is a product I use daily because it just works, and works well.
The Facebook/Twitter analogy is flawed, because those are ad-supported businesses, so the company has a strong financial interest in having users on its own/primary platform, where it can deliver ads. I think that's not the case with Slack (?). But even there, I think the incentive to not support N protocols is not to get the +0.1% revenue from IRC/XMPP users, it's velocity/simplicity in product development, which is worth more money in the long term.
Disclaimer: I worked for FB previously, on Workplace, which is a direct competitor to Slack.