The open proctols for RTC today is webTRC. Zoom does not use webRTC. If the proctols Are open like http then I can build my own client and do not have to use theirs (just like you can your own hacker news app) . Zoom will not use webRTC for this precise reason. If they and all others did I can
choose my client and I can choose a client who I trust and will give my green tick open source or not .
Google supported jabber in chat for a long time , slack supported IRC (both dropped the support ) but when they did you could any irc client in slack or use google chat using jabber with any client
If an open protocols for video are used like email (although not good example for encryption). It does not matter who your service provider is you can verify they are secure , or move to another one .
Today I have more than 10 video conferencing apps on my devices (zoom, Hangouts, meet, Webex , teams, GoToMeeting , chime , Skype, SfB, FaceTime , signal, telegram , ring central and Uber conference... ) because a customer , partner friend or family uses one of those . I have only one email and browser client though, it does not have to open source at all, ppl happily pay for closed source gmail or o365 without worrying will my mails deliver to you while still using official client or client of their choice