Other games also charge you for conveniences. Maybe not more bank space per say, but you are paying for a convenience. Mule accounts in FFXI were the same way.
EDIT: Daily limit, you guys are saved!
Of course it isn't free due to a sub, but they could easily have made retainers cost money period. No base of 2 retainers and the benefits they provide. But since we do get 2 retainers for our sub, 1 more is a convenience. We are paying for an allotment of data on the servers. A small fee to increase the data allotment is hardly unreasonable as it is an extra to your base access. Like a smartphone, even though it is broken out of your phone plan, you have to pay for a certain amount of data, 3gb or w/e these days. It doesn't matter if it is broken out or not, because it is a requirement to have a smartphone on the network. If you want more data, you pay an extra fee. We don't have access to an entire database for storage, we have to share among all other players. Also, if you are going to complain about the armoire make a post about that. It still stores things, it still works, just not in a reasonable way at times.
They are going to charge for EXTRA space. They aren't taking away what you already have. Other P2P do things differently. They also have their own conveniences they may charge for. It really isn't an issue with the armoire because EXTRA space is optional. At that point you have to decide if your patience is worth the small fee to store these 19 items on a new retainer that holds 175 slots. If anything, any empty space is wasted if you plan on paying for a new retainer. If you pay the small fee yet only hold 19 items on your new retainer, the space you do not fill is wasted, not the 19 items that you can't put anywhere else. The storage space is bringing its value for those 19 items, because they aren't in your inventory/armory. Even if the armoire should, and it should, hold those 19 items, it can't right now. The only other place to store it is in your inventory or retainers.