Scrolls are consumable, they will never depreciate. People will buy them on a somewhat regular basis.
Withholding upgrade weapons and shiny stones from IM cause their value to increase.
High demand + low supply= high price. As soon as they definitively say there will never be another upg sale, prices will go up to insane amounts once again.
The company still makes money on upgs through people charging for RPs. Theres always going to be upgrades being sold regardless of what they do. It just comes down to if they're going to be sold at a low price on IM or sold at a high price on EM...