So is the winning strategy here to pick anything but the top dogs in the game and hope they never make the big leagues and start behaving like shit? Mattermost just seems like another risky dependency

You can self-host Mattermost. It seems that is likely what they are going to be doing from the article since they talked about how important it is to own your data.

I missed that part in the article, but yeah - self-host or nothing. We are self-hosting as well, although our group is not a large one.

And the self hosted is, effectively, just `docker up`. Saved us $1000s

We used Mattermost but eventually started getting annoyed by the nags to upgrade in the free version. Zulip is has been far better.

It always felt weird to me that glorified IRC could command such a price premium. Admittedly, a bunch of engineering was put in place to make things work, but it was still just humans chatting with each other for what is probably tiny amounts of data storage.

Anything you can self-host is mostly safe, because at the very least you have access to the raw data and can move elsewhere if you need to.