Phone companies are different in that they were monopolies with special legal status. Slack, on the other hand, is just one of many private companies offering roughly equivalent services.
You can still argue your case, of course, but phone companies are a bad analogy here.
The phone company is the sole-source provider and you couldn't opt for another platform. The more relevant analogy is soda fountains in a restaurant - the provider chose Coke, but I want Pepsi. (My OSS founder wanted Slack, I want discord.)
That's a different medium, although I understand your point: Slack is the sole source provider of communicating on the Slack network. Personally, I don't think this is a problem or should be fixed because of how I see the tradeoffs and side effects. Similarly, I want Apple to run their own app store and not allow sideloading because I prefer the set of tradeoffs that come with that, versus the other reality.
