I agree that there is no point to the permanent durability reduction if it is only optional.
I personally dislike the mechanic itself, at least its current iteration (RNG based, and with massive durability reduction, as you said a flat 5% would be okay-ish).
But what is even more stupid is that there is no reason to repair anything aside from your amor, because you’re gonna go for Legendary Weapons that cannot be repaired but are straight up OP.