Fluff vs. Mechanics

[Addaran]

Just to clarify.

Is the entire druid metal armor argument over the choice of the rules writer to use the word "won't" rather than "can't" in the armor proficiencies section of the class description?

Won't implies some sort of limitation on the character's role playing choices? Can't just means there is some mechanical reason (not detailed) preventing a druid from wearing metal armor?

People are objecting to the suggestion of a rule limiting their role playing choices but have no objection to the rule limiting their mechanical choices ... and ALL of this hinges on the choice of "won't" vs "can't" ??

A bit, but there's more. Won't vs can't definitively is a big offender.

When it's based on morality, like in the paladin's case, they say what might happen when you don't follow the rule ( falling and losing your powers).

Mechanically, other classes ( barbarian, monk) will say what happen if you do wear (heavy) armor.

The game also already showed that they are okay going outside of the big categories ( monk and rogue with weapons). Could have done the same with armors. (light + hide). I don't remember if it's 5ed or previous edition, but i think mythral armor say you can cast spell without having proficiency? Could do the same for the dragon armors.

If it's balanced on druid not having too good AC, it doesn't make sense to also block it from other sources ( race, feat, multiclass). I don't think there's any other class that restrict what you can gain from a feat/race/multiclass (at most it's wasted cause you already have it and can use it)

There is actually a very reasonable mechanical argument to restrict Druid's access to the better medium armor at early levels. Namely, their wildshape. Starting around level 4, and in many games even as early as level 2, there is very little downside to a Druid staying in wildshape all the time during exploration (biggest downside is no speech, and there are ways around that, like developing sign language with your companions).

Which means that they basically bypass the disadvantage scale mail imposes, and, at the cost of their bonus action in the first round of combat, can start the combat with 18 AC with no stealth penalty (or, if they are a moon druid, can be knocked out of their wildshape and have 18 AC with no stealth penalty). No other class has that ability at early levels, unless they roll exceptionally well for their dex or are dex based and have defense as their fighting style, assuming in the first place this is not point-buy or standard array.

At later levels (6 to 8 is my personal assessment, it may vary from game to game) things balance out among all the classes, so it's reasonable to let them have access to other types of medium armor

That's what I would tell my players if they ever questioned the clear rule in the PHB.

That's actually the first balance argument i hear. Not sure if i agree, but it's a very valid reason.
I guess you could always say that a druid can't wildshape if he's wear a medium metal-armor (or just one that gives disadvantage) or that disadvantage from armors carry even in wildshape.