As we speak, I have five slack communities open in my messenger. All but one of them are open to the public.

I'd wager there are more public Slack communities than there are private (i.e. "team chat") ones. But much like Twitter, Slack has a fundamentally different vision for their platform than their users apparently have.

To me this was first made clear when Reactiflux (a massive community around React, Flux and related web technologies and topics) had to switch from Slack to Discord because they had hit a hard limit of members and Slack said they wouldn't increase it because their software and infra wasn't build to handle these scenarios.

Nevertheless open source projects and community builders keep flocking to Slack and Slack doesn't seem to have any intention to clear up this misalignment. They're benefiting from the free advertisement small public communities create but don't want to spend any extra energy providing their services to them as they grow.