Ironically, if anyone is going to threaten Zoom, its likely to be Microsoft with Teams, and only because so many organizations are already on Office 365 and Microsoft seems to be sinking some sales channel hooks into Teams specifically, and arguably Teams is good enough and has most of the same extension points as Zoom does, including its own add on store and API for most major integrations[2][3][4].
It seems Google is having a lot less luck here, first hand experience being that every business I've worked for in the last few years have been Google Workspace customers, yet still prefer to use Zoom over Google Meeting, despite Google Meeting being effectively free with the rest of the Google Workspace product.
I'm still mind boggled that Slack didn't capitalize on video meetings, they had all the technology and know-how, however execution matters like anything else, and I don't think they've been able to entirely execute on adding strong robust video offering to their core product.
[0]: https://investors.zoom.us/news-releases/news-release-details...
[1]: https://www.cnbc.com/2019/06/06/zoom-earnings-q1-2020.html
[2]: https://www.microsoft.com/en-us/microsoft-365/blog/2019/03/1...
[3]: https://docs.microsoft.com/en-us/microsoftteams/platform/sam...
[4]: https://support.microsoft.com/en-us/office/find-teams-apps-d...