You've left out Element, which does provide a similar suite of features with its Spaces. I concede Mattermost and Rocket.Chat. Thanks for the correction.
I think they've had a pretty frustrating experience with corporations freeriding on their work. See, for example: https://news.ycombinator.com/item?id=34129623.
I wish them the best of luck, and hope they're seeing success. Certainly in 2025 Europe should be thinking hard about the risks of a huge portion of workplace communication happening in Cloud-based chat and email applications owned by a few US companies (Microsoft/Google/SalesForce).
Honestly, we've had the same problem that Arathorn highlights in that thread. When I first started, I imagine businesses self-hosting Zulip would pay for support and fund the project that way. In practice, Zulip is too easy to operate for self-hosters to do that, and the better we make the product, the less support people actually need. The support-only business model can work, but I don't think it's workable for chat.