Why is this specific mechanic during Relics Quests still that way?

[UkcsAlias]

Think about every turn-in you've ever done. The reward goes into your inventory.

Your armory and loadout is still just an inventory, where chaning loadout just exchanges weapons in slots.

If the weapon is in your main-hand and you turn it in, what job are you afterwards? You have no weapon. No job. That isn't a "state" that FFXIV knows how to handle. Should it be possible? Sure. I wouldn't be surprised if they are wary about making changes to code that has been in place since at least 2013, if not carry over from 1.0.

Considering the change is effectively instant, you arent a null job. And the turn in doesnt happen when you click ok. As after that click, your duty can pop, and the item is still in your inventory. The only moment is when the game confirms the change of state (quest completion). This is an instant change. If your duty pops, either you have the old or new state. But you dont lose your items.

And for this purpose alone, they could have made a null weapon slot for each job to use in these cases. This is a slot that is normaly not accessible, and never actualy saved either. This way the item can always be moved without issue. (the null slot gets the new item, and then the weapons are changed around as if its a player action). And we know this slot can exist without issues, since we used to have a belt slot.