Seems silly for any druid to be allowed to mercilessly upset the balance of nature without any repurcussions. Any player who does that and then comes back with "there's nothing in the rules..." seems like a classic "rules lawyer" to me. I'm glad such types would voluntarily leave. Props to the op for that. He or she would have the decency to go away in that case.
Then you don't know the nature of the game in its current condition. Druids do not have an alignment restriction like they have had in past editions, just as Paladins no longer have alignment restrictions. The Feywilds are not just full of neutral or good creatures, and there are plenty sufficient evil plant creatures. If you gain your power from the divine, it need not be radiant power. You very well could make a Chaotic Evil Druid, just as you could make a Chaotic Evil Paladin, or a Chaotic Evil Cleric.
Currently there are 5 deities in the Forgotten Realms setting of 5E that could offer Clerics the Nature Domain, and only one of them is neutral, two are good, and two are evil. All 5 of them grant Clerics proficiency in the heaviest of metal armors. Druids can gain their power from a deity just as a Cleric can. They could likewise gain power from the darker powers in nature. Whereas a good Druid may earn the favor of Dryads and Treants, the evil Druid may earn the favor of Hags and Blights. There's also the character that doesn't try to appease the spirits to gain their favor, but rather takes power for themselves. Hags, despite their evil nature, are fey creatures, just as Dryads are, but still also exert control over nature despite their proclivity for death. Just by living in an area they change the very state of nature around them, causing trees to attack living creatures, the air to become poisonous, etc.
The very point of the argument is not an argument of "there's nothing in the rules", but the fact that the game system in 5E is built very contrary to what you're stating. You're implying it's still heavily one-dimensional, where a deviation from the class must have consequences on the class, but that's outdated standards. The game has been changed from that idea for nearly 20 years now since 3E came out . Druids could become good or evil, and Paladins had Blackguard in the core DMG for them to continue having corrupted Paladin powers after falling from grace. In 4E such restrictions went away for good, and it has continued that way into 5E. If a DM can't figure out a way to handle situations without simply taking away player agency or threatening to take their class away, then they simply haven't adapted with the times.
EDIT: Just to note, I'm not saying any one player should be deviating from the play that everyone at their table agrees to ahead of time. What I'm pointing out is that the game system has equal player agency built in for players playing as any class, including Druid, as per the core rules of the game. That's what makes the metal armor taboo a busted "rule". If every player wants to play a certain way, I totally agree with removal of the player that wants to deviate from what the rest of the group wants in a way that's disruptive. However, I do not agree with the removal of player agency from a single player at the table simply because the DM believes their class must always make certain decisions, even going so far as to rule possible actions that any character could perform to be impossible.