Workiiin' aaas desiiigned It is in their interest to keep people subscribed every month forever, which means a constant rotation of phasing out materials and replacing them with new materials, a goal which is not served if people own permanent physical books

I don't think so entirely. The reason why you would want to keep the subscription, especially the Master tier is to share the digital books you have and to also have unlimited character sheets. Though I do not trust they will support everything on there forever.