Not a Slack employee or anything, just love the software.
Seriously, the only thing that could convince a company is either lower admin cost (vs Mattermost) and way better customer service when something fails. (vs Discord) Pretty sure big co mc corp wouldn't like a potential competitor running its comms.
Additionally a strong competitor is as always email. Perhaps enhanced with scheduling and mailing lists. If you don't like to set it up, you could pay Google for example. And both Google and MS are improving their chat/IM offerings.
They can and will undercut such a start-up instead of buying it, unless they see enough value in an acquihire.
Companies that make heavy use of office would find it very difficult to move over to another office suite (if they could find one that meets their needs), whereas moving off slack to one of the many alternatives, doesn't seem that hard.
Might be an overstatement. I would say let's see until they innovate on something that people like and would pay for. Until then, I wouldn't call them a failure until they actually run out of business