But Microsoft is the most likely buyer, it's been on a spree with integrating best-in-class enterprise products/productivity tools into Office 365 (Sunrise, Accompli, previously Yammer). Slack fits into that mold, though the sale price would probably be orders of magnitude higher than those acquisitions (likely upwards of $10 billion, possibly much more). At that price tag, it might be untenable.
Not sure what Slack's medium-longer terms goals are, honestly. Their valuation is so high that only megacorps can afford them, but there's still enough risk that public markets might be skeptical (their revenue growth is astronomical, but they haven't been generating revenue for THAT long). In the interview, Stewart mentioned market saturation eventually limiting their crazy growth numbers, but I'd imagine they have a ways to go, given that the total enterprise market has to be well more than 200,000 total seats. Exciting times for them.