I think we're mostly on the same side of this argument. I give style points for druids who choose to not wear metal armor, but I think that's all it is, style. And style points don't really count for anything outside of good story telling, and if there's no good storytelling to go along with with metal armor prohibition then it feels pretty empty and pointless.
The poster in the thread with the mariner's armor that made the druid feel icky doesn't bother me much. But personally, I'd still allow the character to wear icky-feeling armor if that's what they wanted to do, and they'd suffer no mechanical penalties for it, but I'd hope that we could not just forget about it, but use it as a minor element in the story, for moving along a sideplot, or as comic relief or whatever.
If I want to start a game as a cleric with scale mail, and then take level two in druid, what happens? I mean, ideally, to take a level of druid I should be associated with a circle or somehow acquire a mentor or whatever, but in reality, it's a game with other players who all have their own storylines and character development that they want to realize and there's limited table time. I know there are probably many ways to go about it, but what do you do with, for instance, a cleric that wants to continue to be a frontline fighter, that has taken a druid level, and has not yet had time to go questing for special armor? I like all the individual pieces of the puzzle. You can tell the druid initiate that they have to strip and they get an armor penalty until the party can get around to going to kill an ankheg and then finding an armorer to make them a half-plate... You can drop an ankheg half-plate in the next pile of loot? I think it feels best to say that the character continues to do what they are doing. Maybe at some point they meet up with a group of druids that give the character the business for wearing gross stinky metal, or maybe they just feel increasingly uncomfortable in the metal armor. Maybe it actually gives them a rash.
I'm starting to ramble. I'll try to wrap it up. I would not drop everything to get the new druid new armor. That seems disrespectful to the other players. I would not give the new druid an armor penalty for taking a multiclass when I fully intend to get them whatever armor they want in the future, and I wouldn't just drop a chitin halfplate in the next loot pile because that's incredibly cheesy. I'd let the character keep wearing the scale mail and I'd work on the roleplay to get from druid wearing armor they don't prefer to armor that they really like, because that feels like the best story. But as always, it's all going to be different depending on the table.