I 100%, no, I 100,000% agree with this. But here is the thing.
If a player doesn't care about the Druid metal taboo... then they aren't going to care. No matter how many rules and restrictions you put on them, they aren't going to care. At best, you just annoy them.
So, why not make it a choice? Why not limit this to a choice made by specific druidic orders? There is a reason the term "orthodox" exists. Different people emphasize their beliefs in different manners. If your druid wants to emphasize the importance of not using metal to defend yourself, that's fine. If your druid wants to emphasize a disgust for metal in general really work to remove all of it from their life, that's fine too.
But it should also be fine that my druid cares more for other aspects, and doesn't consider the metal defense issue to be as important. My druid should have that choice. And if that means that a bunch of other druids turn their noses up at them... sure, whatever. But I don't agree with removing that choice from all druids, across all time and space. Not when it is so easy to see how this taboo wouldn't apply to many, many different druids.