Yeah, it's a badly designed rule. On top of the OP's complaints, which are entirely on point, it ignores the obvious question of "Can I get nonmetal versions of the armors I'm proficient with, and if so, how?" If I kill a dragon and have armor made out of its scales, is that hide armor or scale armor, and what does it cost to have made? If it counts as scale armor, are there other monsters whose epidermis can be made into scale armor and that are a little easier to kill than dragons? Is there any way to make half plate without metal? What about studded leather, do the studs contain enough metal to constitute metal armor, and if so, are non-metal studs feasible?
Normally these are questions that come up rarely and aren't particularly important; the DM can handwave an answer and move on. But because druids are proficient with lots of armor that will cause them to explode if they wear it, every druid needs an answer to this, which means it ought to be answered in the rules.
They should either have limited druids to proficiency with padded, leather, and hide, or scrapped this rule entirely.
Normally these are questions that come up rarely and aren't particularly important; the DM can handwave an answer and move on. But because druids are proficient with lots of armor that will cause them to explode if they wear it, every druid needs an answer to this, which means it ought to be answered in the rules.
They should either have limited druids to proficiency with padded, leather, and hide, or scrapped this rule entirely.