> It's easy to replace Slack the tech, it's hard to replace Slack the app that people are used to when they say they want a chat app for their company.

Well, that's not true. That is easily refuted by it having been replaced at least once.

> It's like saying Windows is not difficult to replace. Technicaly true but practically false.

While you may believe that Slack offers something that other solutions lack, that simply isn't true. I know of two companies which have replaced Slack, one with a commercial offering, and the other with an in-house solution. Rewriting most of Slack is a week-end project.

Slack, over time, actually locks a company more and more into their ecosystem, since to have history you must pay them. With many other solutions, including 50 year old email, messages can be stored locally and filtered as is beleived to be appropriate by each user. If all you want is a chat bot with channels, then use Slack, and throw it away when you're finished.