So, this restriction against metal armor for druids is primarily a "fluff thing" and... like most D&D "fluff things" it wasn't very well thought out, and it took a very narrow historical/literary inspiration and applied it very broadly in a "one size fits nothing" sort of fashion. (See the history of clerics and bludgeoning weapons, for example.)
Problem is, if you just shrug and allow druids to wear any armor they want-- you're going to see all your druids in half-plate automatically, and we don't want that, not because it's "overpowered", but because it's weird and it doesn't fit our idea of what a druid is supposed to look like.
Personally, I think D&D could stand to do with a crash course on spiritual taboos that aren't based around punitive consequences, but the character's inability to make the decision to break them. A druid will not wear metal armor. Wearing armor doesn't make them an ex-druid, they have to become an ex-druid to make the decision to wear metal armor. Maybe the one (and only) druid taboo isn't a great example for this, but this is ripe territory for warlocks.
Problem is, if you just shrug and allow druids to wear any armor they want-- you're going to see all your druids in half-plate automatically, and we don't want that, not because it's "overpowered", but because it's weird and it doesn't fit our idea of what a druid is supposed to look like.
Personally, I think D&D could stand to do with a crash course on spiritual taboos that aren't based around punitive consequences, but the character's inability to make the decision to break them. A druid will not wear metal armor. Wearing armor doesn't make them an ex-druid, they have to become an ex-druid to make the decision to wear metal armor. Maybe the one (and only) druid taboo isn't a great example for this, but this is ripe territory for warlocks.