I'd say inertia. It's there, easy to get started with, and 10k messages sounds like a lot. Until you actually start using it. Once you start hitting their limits, it's too late to start looking at alternatives.
Building an alternative isn't that hard. There are plenty of them out there that are completely free and even open source (I speak from direct experience here, we built one of them).
But, once a product has the inertia, it's very difficult to stop them. We've seen this many times before. DOS, Windows, Playstation, Git, VHS, Microsoft Word, Ethernet, QWERTY keyboards, etc.
In some cases, the inertia of one product leads to a forced standardisation which can be a good thing. In other cases, not so much. I leave it to others to decide where Slack falls on this line.
Building an alternative isn't that hard. There are plenty of them out there that are completely free and even open source (I speak from direct experience here, we built one of them).
But, once a product has the inertia, it's very difficult to stop them. We've seen this many times before. DOS, Windows, Playstation, Git, VHS, Microsoft Word, Ethernet, QWERTY keyboards, etc.
In some cases, the inertia of one product leads to a forced standardisation which can be a good thing. In other cases, not so much. I leave it to others to decide where Slack falls on this line.