All else not withstanding, how exactly is the game supposed to know whether or not a scroll have became useless?
No, it's not a "magic trigger" that "just knows", you have to think about it from a programming prospective, HOW to do it if it is to be done.
Does it track how many have been dropped already? sure, that could be done, but what if someone, for one reason or another, sells one? It would complicate the problem if the game have to track item sells, especially in batches.
Does it track how many equipment have been upgraded? While, that is certainly a workable idea, but it present the following problems:
1. there could still be excess while people are farming for materials.
2. it would require either a full equipment check EVERYTIME or a explicit boolean assigned to each item that need to be stored in the database. It would be a nightmare to have an ever changing table for every new scroll, so a boolean array or a int bit switch format would be most feasible, but it might still run into expansion problem eventually. And to make such a boole*eful, a full equipment check will need be performed after every equipment upgrade, generating potentially noticeable lag (thou not if it's optimized but... yeah... optimized browser games, haha)
Would it be nice?
Sure
Cant it be done?
Yes, thou maybe not perfectly.
Should it be done?
I'm not sure.... I'd rather have some more new features instead of worrying about such trivial matters that doesn't have any significant impact.