They paid an incredible amount of money for Slack because of what Slack is. In my opinion, it doesn't really matter that Slack ceased to exist (which I agree de jure it did) because de facto the Slack culture still exists as a minority now. If anything, the incentives at Salesforce should be to preserve Slack, not whittle it down, because they thought Slack was worth billions of dollars. Is Slack's technology really worth that much? Maybe, maybe not. Is Slack's customer base worth that much and do they have a big enough moat that they won't start evaporating? Maybe, maybe not. More likely the value at Slack was created by the sum of its parts together, including culture.
Maybe you can make the argument that Salesforce thinks they bought a company that was worth less than the sum of its parts and that they can unlock value by making changes, but the acquisition and "culture clash" imply to me a big corporation throwing money at something they don't really understand.