You can put over 500 people Mattermost for $5/mo. And it's easy to get tightly integrated to your corporate environment - and even allow clients as temp-guest accounts for support.
Coding the custom solution around Mattermost cost less than one year of Slack and in year three we're still happy with it
Completely agree -- Mattermost is a great option for anyone. I'm shocked more people haven't adopted it. People need to get real. Slack is absolutely bonkers expensive. It may be the case that SV companies are happy to pay this price but you'll limit your market to SV. Slack is definitely finding it harder to grow -- they'll end up like DropBox.
Coding the custom solution around Mattermost cost less than one year of Slack and in year three we're still happy with it