people seem to forget that each large patch allows more stuff to be put into the armoire.

Anyway, the easy answer is to dump/stop hoarding all that unnecessary stuff. Turn it in for GC seals, vendor it* or toss it.
*In the early days, I used to hold on to mats I might use. However, with the way the economy was going, it was better in the long run for many items to vendor it early and buy back later. You can even make gil in some cases as the item may have dropped below vendor sale value on MB.

However, for those who somehow manage to fill 350 slots on their retainers, I agree that additional storage (not a retainer) should be purchased with gil or even a gobbie bag type quest, rather than real money. An additional retainer (due to the ventures, whatever) should be optional with real money--akin to mules, etc. of ffxi.

Of course, SE could say "Screw you, we'll just make more cowls / tin cans to save you an armor slot!"