I'm thinking Slack's issue is their free version is too good and the paid version is way too expensive (per user pricing gets scary) I know in my experience we don't have any trouble using free and we have too many people to make a jump to the paid. I think Slack needs a "middle ground" solution where small-medium businesses won't balk at the per user pricing.
How can you be valued at $5 billion and not be a functioning business? Investors will never see that money.