Spot on, the 10,000 free limit is much more limiting than many people think. I've seen very small businesses of even 3-4 people lose tons of institutional knowledge just because they wouldn't pay for slack. With larger tech groups you can start losing information after a few days. And the larger the group the less likely they will pay for it. I like the UI of Slack but am happy when I see tech groups using Discord instead.
I have more thoughts here if anyone is interested, including alternatives, "Why tech groups can use Slack for free but open source projects and businesses of any size shouldn’t": https://medium.com/@gabriel_wilkes/why-tech-groups-can-get-a...