If over-enchanting destroys items
Over-enchanting items comes with a potential risk that the item decays or is destroyed if a safety margin is exceeded. This system is subject to testing.[10][11]
How can players know that the artisan indeed lost the item and not just pretends to be so?
I assume that the process of enchantment takes time and will not happen in a few seconds, while the customer is watching.
But if it is indeed a visual process the player can observe, then he might see how his weapon breaks into pieces.
Edit:
Some players do not like the RNG associated with the enchantment.
Should item sink take place through a different game mechanic?