There was an interview around the time Heavensward released in which Yoshi acknowledged that the addition of a lot of extra items and three additional jobs with the expansion could make our current inventory/storage difficult, but that improving the game's storage systems was a lengthy project that they wouldn't be able to complete in time for the expansion, so as an interim measure they'd make additional retainers available.

I (and apparently a number of other players) thought at the time that meant we'd get another free retainer, but it turned into simply expanding the maximum number of extra paid retainers we could buy. But regardless of that part, it was supposed to be an interim measure until SE could fix their woefully inadequate storage. We still, nine months after the expansion came out, haven't yet seen this real storage improvement that they were supposedly working on a year ago.