So...don't use it. Honestly, there are other people here that will see this link and go "hey I want to add this feature in this" and the tool becomes better. Once it does I am sure you're gonna come back and leech on it.
What I'm saying is that "pull requests welcome" is a really lazy way of telling someone to support themselves. That's fine if you have a unique set of needs, in that case you should be building things out specific to you.
For something like chat, most people have a very similar set of requirements, paying Slack a couple of dollars per month to handle it is awesome. Would it be cool if Slack were opensource? Maybe, but that isn't why people would use it even if it were. It works really well, there is a team behind it that is paid full time to develop new features, and we can focus on building our own company instead of using bandwidth building tools for basic functionality.
That doesn't mean there isn't room for improvement, but the places for improvement in Slack aren't that it isn't open source.