Since giving us more inventory/storage options seems such an impossible feat...

[Itseotle]

If you haven't figured it out by now, the reason for the limited inventory is to cut costs. We get 300 slots in our armory, 100 slots of inventory, 200 slots of retainer space (for free), 15 slots for key items, 10 soul crystals, 14 equipment slots, and 18 slots for shards/crystals/clusters. That's not counting things like Mailbox, Market Board, Chocobo Barding, and retainer equipment space. That's 657 inventory slots per character. And its not as simple as "Has item: 1, Does not have item: 0". Your regular items have data on condition, spirit bonding, dye, creator (for signature), glamour, and custom stats (like aethereal gear and relics). What this means is the memory required to store all this data is much greater than the common player may think, and thus, the server costs go up the more inventory space you allow. The two paid retainers are a way to cut the costs somewhat, by adding a fee for the memory your using, and whether or not you think inventory costs should be covered under subscription fee is an entirely different topic of discussion.