I agree Heroku has in this way been a model acquisition, but in this instance I can't see Salesforce being able to make those assurances – they likely see Slack as a key interface for their products and services, and integrating the two to support that vision will require oversight.

I can still see a world where Slack is a tool that continues to operate and serve its customers, similar to Heroku post-acquisition, but Salesforce augments their services with Slack tie-ins (and vice-versa).

Short: there's a middle ground I think they can strike.