Chaosmancer
Legend
How can you not get it? It is terrible game design to make players choose between archetypal theme and a mechanical benefit. The rules should do the exact opposite!
Because there should be no mechanical benefit. If the mechanical benefit is that huge that it cannot be overcome by a passion for the theme, then you should provide the alternative types of armor that were traditionally available for Druids.
Then both the theme and the mechanics are satisfied. I'd just remove the non-metal theme, because I think it is nonsensical but there you go.
I mean, the list of fairly natural creatures that breastplates and half-plate could be made of is decently sizable.
Ankheg
Basilisk
Behir
Bulette
Chimera
Anklosaurus
Dragon Turtle
Gorgon
Grick Alpha
Hook Horror
Hydra
Naga
Otyugh
Piercer
Purple Worm
Remorhaz
Roper
Roc
Shambling Mound
Troll
Umber Hulk
Abominable Yeti
Giant Crocodile
Giant Scorpion
And that is just the MM and just the creatures whose natural armor is 14 or higher. Let construction and reinforcement be a thing and few other options open up, like Giant Crabs.
Additionally, this isn't something that would somehow "break a classical setting" because again... people do this stuff. If I went down to a swamp area like the Everglades and I DIDN'T see people with stuffed gators or gator skulls as decorations anywhere, I'd be confused. If there are Giant Crocs whose hide could be used as armor, then there are going to be people using that hide to make things. Like armor. IT would after all be cheaper than importing armor.
And if the druid is really interested in hunting down their own Giant Croc to skin and fashion the armor from the hide, scales and bones themselves... then more power to them, but if they aren't, just making it a thing they can buy off someone, maybe an old hunter who doesn't need it anymore, isn't somehow destroying your world-building.