Unity made a specific committment in their terms that if those terms changed in a way that disavantaged existing developers, those developers could carry on using the existing annual release under the old terms, and also that they would notify developers of changes to their terms. They then sneakily removed that committment and almost immediately imposed this licensing fee for use of the runtime, when one of the specific advantages of Unity they promoted was not having any such fees, and retroactively applied this to new installks of all existing games. It was shameless and some company offering open source software could jsut as easily retroactively decide it wasn't open source at all.