Slack is an outlier because you don't actually need to communicate with everyone in the world on Slack.

But for others, it would be completely unreasonable to ask your acquaintances to pay to install Skype to talk to you. And you probably wouldn't pay for Skype if it didn't interop with whatever other people were paying for.

In a world where switching costs for consumers are so low, interop becomes less important because you can ask your contacts to install an app if it's useful enough.