I'm going to disagree here. It's entirely possible to work effectively with others via mostly-asynchronous communication. Open source projects, as you point out, can be a pretty good example of this. I'd further argue that for some (I suspect many...) people work much better when they can have long stints of focus without fear of interruptions.

Two-minute responses on Slack absolutely is a model that can work, and perhaps it's the easiest transition for people who've been working in a "teamy"/open-plan on-site setup. But I'm also convinced that it's not the only viable setup.