Slack's business model has always been that you give them all your most critical data and they sell you access to it. This is basically the business model of the traditional kind of ransomware, before people got better at making backups.
You probably should expect large bill increases over time from ransomware-as-a-service companies like Slack. Not all of them—people are capable of behaving decently—but probably the nature of the category is such that you should expect it of most of them.
When switching providers is impossible, the pricing of maximum profit for the provider is the pricing where the buyer is exactly zero. Slack presumably doesn't have quite enough information about their clients' businesses to calibrate this exactly, but if they can approach it approximately, they'll make a lot of money; even though they drive some of their customers out of business, those losses are compensated for by the higher revenues from their surviving customers.
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