Why So Many Key Item Slots?

[Xieveral]

Why not just open up those extra grids and have key items and regular items share the same storage? It's not a perfect solution but at least that extra storage space won't go to waste.

Because it's probably theoretically possible to hold that many KIs at once if you have the max capacity of active quests. It's the same sort of reason there is enough armoury chest space for every job's soul crystal + 3 specialist DoH soul crystals, even if a player has no intention of actually obtaining them all.

The devs still plan to introduce new classes in the future. Why would they do this if they weren't capable of expanding soul crystal storage? They're clearly able to add more storage space since we got some to accommodate for the new jobs and DoH, they're just stingy about it unless it's in the form of a cash shop feature.