Yes.  The price you subscribe at will never change for the same tier of service.  Even if the pricing for the app is raised significantly for future subscribers, your pricing will never change from that which you subscribed at. 

And when we say on the Home page, "Your annual subscription fee for all features currently offered will never increase", the term "currently offered" applies to most features that we will continue to add to improve appearance, searching, hosting, editing, etc.  The exceptions to this would be if we add features (currently unplanned) that have separate or higher pricing, such as storing your photos and videos, printing books, hosting ebooks, etc.  In cases like these, your price may not include these additional items and services.