The problem here is that companies artificially limit integrations, so it's impossible to exchange messages between different providers like how email works.
We really need some legal requirements similar to "right to repair" for machinery. We need "right to integrate" for software. I don't know how you'd pull it off, or how much support for integrations would be "enough" but it would allow competitors to cross these network effect lock-in moats that large players are able to build.
I’ve always thought that the proper competitor to Slack/Twitter/etc… was a protocol not another service. Protocols enable competition, services just shift the market to another service.