"possible solution on how to make the metal even more useful"

Just imagined how it could be done (though I was thinking about the same theme* a lot, during veeery looong time):
1. Make it so that the crafted items would give any special stats on any special map / gamemode (new or old) (For example, on MvM: the crafted weapons give extra cash per destroyed bot).
2. If the item has been crafted by its owner (the person who equips it), the bonus is even bigger (or it is even changed in a quality, not just in a quantity).
This way people would have choice to buy the key and open the chest OR to craft the hat / weapon to use it with the (additional) bonuses. And the metal would have more options to be used!
(Again: on special map or in the gamemode, not during the vanilla gameplay!)

(*Of course, the item_origin and/or the item_quality could be used in some user's modification, to make the gameplay more variative. "Crafted" items (even cosmetic ones) could have 1 kind of bonus, and the items "Found in crate", or "Gifted", would have another bonus, in very specific gamemode and map. On that special map, even hats would give special bonuses to their wearer.)

Source (my post in the thread "Let's talk: Keys", on SPUF)