Even assuming you already have every other minion in the game (well done!) any inkling of 'completionist' in you means that you will for certain be buying Endwalker anyway... so why would you not preorder?
This isn't like new game releases, where marketting gets people to preorder an unfinished, unseen product. This is a mandatory expansion to an ongoing MMO. You don't have a choice. If you want to play FFXIV, then you're going to buy Endwalker.
What else are you going to do? Hang out in Eulmore at level 80 for the next 5 years?