How is this a fault of Slack?
Now, Teams would have handled this better.
One team = make a team = make channels in the team
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.