They already gave you more inventory space with the choco saddlebag last time, this was kind of easier to do and a clever workaround because it doesn't follow you to the instance servers so those don't take any hit in performance there. but the overworld takes a hit. From a simplest explanation, the instance servers do much more, like the overworld doesn't need to track your movement in the housing districts as much as the instance servers needs to in Eden Savage to see whether you are in the constant AoEs or not kind of thing.

Considering YoshiP's last comment on glamour plate limitations a month ago and it being server capacity issue, it's the same thing here

MMO inventory is in all actuality a difficult process to manage, it's not so much spaghetti code people like to think of it as. Look at it more closely. All of it is server-side and is "following" you everywhere you go. You want to use that potion you put on your hotbar anywhere you go. That hotbar has to check the item exists and quantity everywhere you go. That's the simplest explanation, every toon takes up server capacity, and if i made your block on the server a little larger with 50 more slots, and I also want to get more people logged in the game at the same time. Something has to give. Capacity doesn't come from White Mage's thin air.

You can imagine where the priority is going to go next. Yes, freeing up the belts gives them back some capacity but they already accounted for most of it, along with stat squish to lighten server math and loads. It'll be a combination of things over time if they were to give more slots.