I don't think the messaging platform should always be on-premise. You can consider running it on premise, but it's not an obvious decision.
I worked on a couple of teams, and somehow no IT team could make the chat apps run reliably: we always had disconnects, terrible loading times, lost messages and we had to jump through hoops to enable the most basic integrations. No regular updates, outdated apps that are years behind in looks, features and integrations.
Whenever we just simply used Slack, it was snappy, reliable, and it was easy to set up connectors for different external services.
I worked on a couple of teams, and somehow no IT team could make the chat apps run reliably: we always had disconnects, terrible loading times, lost messages and we had to jump through hoops to enable the most basic integrations. No regular updates, outdated apps that are years behind in looks, features and integrations.
Whenever we just simply used Slack, it was snappy, reliable, and it was easy to set up connectors for different external services.