I'm fairly confident that his comment was relating only to the ranked season titles as this is going to be the last ranked season for Ishgard. They're giving people a lot of forewarning because people are pissy and complained a lot when they tried to change the title to "Saint Ascendant" and so they feel like they can't just get rid of the chance to obtain the Saint of the Firmament title without giving folks a heads-up. Unless they do away with Diadem altogether with this next patch, which I highly doubt is the case, you will be able to continue working towards the Pteranodon mount.