Pay for your inventory.

[Adire]

Other than the increase in retainers which translates to more ventures going out, where is the expanding functionality coming from?

(Much needed) Storage.
Greatly increased sales capacity (which translates into market domination).

Whoever pays SE gets to not have their inventory crammed by the onslaught of expansion items. There's no alternative to retainer subscription as increased storage, players will have to inconvenience themselves and make do with the already quite pitiful storage.

And the ability to pay SE more and get market domination abilities over those who don't is pretty self-explanatory. On Balmung we already have someone who floods the market with certain items (like grade 1 glamour prisms). Now they can do that even more.

Real world wealth inequality being brought into the game is a bad idea, especially in a sub game. That is why I do not like these increasing cash shop updates.