Slack does exactly this, but then it makes business sense for both parties because they can tell a company "buy licenses for all your employees, but don't worry

Slack does exactly this, but then it makes business sense for both parties because they can tell a company "buy licenses for all your employees, but don't worry we won't charge you if they send less than N messages a month". When dealing with individual consumers the service doesn't really have any incentive not to charge you.