It's very unlikely that you will ever lose access to a title midway through it. We know how frustrating this would be, so we do whatever possible to make sure we never remove a title once it has been added.
However, it can happen that certain titles are removed on short notice due to contractual reasons out of our control. In these cases we work with our content providers and publishers to reinstate the titles as quickly as possible while also renegotiating agreements to add titles and expand our library with even more content. Thanks for your patience and understanding!