You misunderstand. It was not the open source purist that chose Slack, it was the wave of popular projects all on Slack the open source purist could no longer resist joining. The alternative to Slack in their case isn't IRC, it's doing another line of work as all the projects within their chosen area of expertise are on Slack. Same is true of my other examples, there's a tipping point where your principled stance forces you to be an outlier, completely irrelevant to the world.
edit- In this case, the open source purist has to join Slack if only to beg people to go to IRC, a move that is at that point more energy than the community will think worth the purists principled gain.
edit- In this case, the open source purist has to join Slack if only to beg people to go to IRC, a move that is at that point more energy than the community will think worth the purists principled gain.