I found Slack not productive in the business context. Took away too much time from work.

Teams is the same in that regards but as it aims for a large use case - video conferencing, integration with email,... - it feels more natural.

The point I do not get is what Salesforce has in mind to justify the costs. Existing users? No, SFDC has access to those themselves. The product? No, that can be built easily - an app like Slack is no rocket science.

No idea, but SFDC knows what they are doing, I am sure.