Hello @BenWhately,
This is obviously a disappointing decision, but a business is a business. I will greatly miss your product once it is gone, and would have gladly paid to keep using it, as opposed to being a lifetime member.
This being said, if you are serious and honest about allowing users to move their content away from the community courses before all this precious work disappears, you can open-source part of it, as you hinted to before when mems were removed.
The most precious thing being the content of the courses themselves, you can at least provide a read-only access to your database backend with enough details about its schema so that users can extract ALL the data related to their courses with proper SQL-like requests, or whatever proprietary database you originally put in place. The community contains enough software developers to help other users export their data.
Everything else is window dressing at this stage.
All the best for your future projects, and best regards,
Hugues Mercier