I though their growth was just fine. I think the fact that you do not need to host it yourself is a force and it also lets them concentrate on exactly one product. I can imagine that providing a supported on premises platform must increase your support team size considerably.
From what we've seen at Mattermost (open source Slack-alternative, https://mattermost.com/) supporting on-premises deployments with an open source community is actually pretty awesome.
We have over a thousand people on our forums helping each other out, people are building installers for Mattermost in Puppet, Chef, Ansible, Docker, etc. (just web search to find them).
