That's how you know if a product is for enterprise or not.
If the license model decouples actual use / utility from revenue, then it has probably been designed that way and is targeted at enterprise.
The Slack model works for applications that are constantly being used and is the center of a persons job. Like chat apps, code editors, creative tools aka Creative Cloud, Audio Tools, etc. It doesn't work for tools you only use sparingly. Like backup/recovery tools, database tools, profiling tools, etc. If you applied the Slack model to the previous set, it would not be sustainable. Your paying for value not use.
How users are counted towards billing?
"Once users are created they are automatically counted towards billing even if they don't accept the invite or ever login. A user must be explicitly deactivated, deleted, or removed from a synced user directory (if you have Google sync) to not count towards billing. Learn how to add or remove users."
With slack, they are strongly incentivized to keep the product engaging enough to hold onto users.