Earlier quoted context omitted.
There’s no money to be made in creating and building on open protocols. Without money to be made there’s no funding. Without funding there’s no way to create great experiences. Without great experiences nobody will use the thing. Pretty straightforward, really
On the other hand, I don't think anyone accused slack, with multi-megabyte webpages and even worse mobile app, of being a great experience. I think I used it once, and said no more.
Overall Slack is a net UX improvement over everything that came before.