2 cents. I think a lot of folks like Slack, evidenced by it's degree of usage (I can get stats for you if it is of interest).
We also had available to us (and might still have) Hipchat by Atlassian. But we didn't have much interest shown in using it. Hipchat might meet the technical requirements (haven't investigated lately), but if nobody uses it, then it's not going to be effective. It's not just a tool thing, it's a culture thing and Slack seems to be doing pretty well on that front.
I do agree it would be more open a little bit more useful if we got the full message archive. But I don't think it is worth the cost. And there could be a cost in trying to move to a different tool.
If folks do want to look at an alternative chat tool, let's inventory the features in Slack that are making it popular with our community.
Personally I'm happy-enough-with-Slack not to want to be investigating alternatives.