I’ve only been in the game for a year, but I’ve all the DoL and DoH, plus two jobs at level 50. I also use retainers quite a bit.

My sense is that more tiers of items are introduced with game expansions, but the inventory remains static.

In my situation, I like to have my tiers of materia organized in columns. So if I have gathering IV materia, I know to slot it into the fourth row. And I maintain a collection of low-tier materia, so that I have it on hand when crafting for other people.

But I see that there are now VIII tiers for materia, with inventory panes having seven rows. So eventually, my arrangement into columns won’t work because there aren’t enough rows.

So my suggestion is that, for the character and retainer inventories, that the number of rows be incremented when an expansion introduces a new tier of crafting items.

Not everyone would organize things like I do, but the principle is to increase available inventory when the game has technically opened the possibility for needing more storage.