I agree that the alternatives should be researched. I've noticed the unresponsiveness of rocket chat myself. SO I will certainly give your Mattermost install a try.
Interestingly, we face the same dilemma at my office. We are using Slack but are really annoyed of the history limit of the free version. We won't buy the licences. T0o expensive for the number of users and their activity. So we are considering swapping it with Mattermost .
Joined :)