Items not requiring repair, doesn't necessarily mean items must have decay.
Perhaps there is no item decay, as well as no durability (so no repairs). And the way sink will work is by limited storage capacity players will have and will not be able to store millions of items.
Then players will need to dismantle items to percantage of base materials, because materials will stack and be usable to craft other things.
This percentage of returned base materials does not need to be high. It can be, for example, 20%. This will sink the materials.
Also perhaps re-enchanting an item will be so expensive that it will be more worth for a player to simply craft new, and enchant it from scratch.
Add loosing of gear, through PvP, people breaking stolen items, and you have efficient sink.
Item durability and decay arent the only ways to create that sink.