The only threats I can see to their growth is WFH disappears (highly unlikely) and Zoom etc. copy them. Otherwise they seem to have a bright future ahead of them.
I think WFH will become a valid - not just pandemic - category requiring businesses to be ready for a "hybrid" model. Software will be needed to support people that will be WFH and the people at the office. I think that's the big opportunity.
I think they're at more risk from Slack and Teams copying them. If your company is all-in on Slack, it becomes frustrating to have to leave it for any other kind of work communication (email, Zoom, etc.)