I've talked to Yoshida about this last year at Tokyo Game Show and I made a post about it at one point, but I'll say it here too.

Q. When can we have more inventory slots? (Such as larger retainers slots/more retainers)

A. We are very limited in resources for data. Resources are mainly being used for other content. Also the more items that you carry/ the larger your storage becomes, the more strain it creates on the servers and resources, which could potentially lead to longer load times and heavier server loads.

I am one for hoarding equipment in games, I like to collect them all. But if that means reducing core gameplay, then I wouldn't want that. The developers are working very hard to create a great game and I would rather they spend their resources on core content.

Consider paying extra $$ extra to pay for their extra work to provide you special treatment and tipping them for their next big server upgrades. I'm sure one day when things become more stable, they will increase inventory slots.
But then again, if that time comes, soon after that we will still ask for more slots. In a sense it's better this way right now.