> Maybe the competition caught up and it doesn't stick out as that special anymore.
Exactly. Slack's biggest benefit was "chat with persistent history, the ability to message offline people, and notifications". That's table stakes now.
IMO the main reason Slack became successful is the massive number of pre-built integrations that were (and still are) available even on the free tier. It meant that you could use it as a message/log/notification aggregator for the various pieces of software you were already using. Furthermore, those messages/logs/notifications immediately became searchable so you could for example do a search for <customer name> and have ALL the stuff related to that customer show up in the results (with links to the source applications).
