Over time, a few dozen or so Slack instances popped up.

These were eventually corralled by central IT and merged into a single enterprise instance, but this just magnified the messiness - overlap between channels/workspaces, vastly different usage habits by department, etc.

Most of this happened pre-Pandemic, at a time when collaboration tools were 2nd class citizens, and nobody cared too much about the sprawl.

Now, Teams would have handled this better.

One team = make a team = make channels in the team

On the subject of Teams, if you want to see something worse than dozens of Slack instances merged into one, it's what happens when those are subsequently migrated to Microsoft Teams, with one team created per Slack channel. Not recommended.

Also not Microsoft's fault. The effectiveness of these tools depends highly on the implementation.

Slack means silo walls break down, channels are formed and disintegrate by people with stuff to contribute in an agile way, rather than a team structure which takes years to change.

That will happen even when starting with a single instance. But that's also going to happen in real life. You can only embrace it...