I'm not so concerned. I think the only thing that would kill Slack is financial factors (financing if it turns out people don't want to pay for it anymore) or bureaucratic factors (if it gets acquired and then mired in corporate stupidity). I think Slack did the favour of proving to the world that people wanted something more than IRC or Skype.
If Slack dies for any of the above reasons, that won't cause people's need for a solution to disappear. I imagine Mattermost would survive just fine. If Mattermost dies, it will be because something came along that's better, similar to how I started out with ICQ, then jumped to MSN Messenger, and now I'm on Whatsapp, WeChat, Kakao Talk, Slack, and Mattermost. Same for Slack. If Slack dies because of better competition, perhaps we'll shift to that away from Mattermost. Who knows?
Gah, how is it possible that the number of chat clients I use keeps growing?
If Slack dies for any of the above reasons, that won't cause people's need for a solution to disappear. I imagine Mattermost would survive just fine. If Mattermost dies, it will be because something came along that's better, similar to how I started out with ICQ, then jumped to MSN Messenger, and now I'm on Whatsapp, WeChat, Kakao Talk, Slack, and Mattermost. Same for Slack. If Slack dies because of better competition, perhaps we'll shift to that away from Mattermost. Who knows?
Gah, how is it possible that the number of chat clients I use keeps growing?
edit: typo