If Google had bought Slack, the free tier would almost certainly have stayed. Of course the compromise would be that you would be served ads and you would have to agree your Slack messages would be used to target those ads to your specific interests.

Salesforce is where products go to die. Slack will eventually get integrated in their Frankenstack and lose functionality. Salesforce can afford this strategy as long as they can keep charging $$$ for their CRM software that genuinely deserves its own circle in hell.