So why not compare druids to monks and rogues? Why you have to compare it to those who are the best? Do druids need to be best at everything? That doesn't sound balanced...
Rangers are the best AC?
I'm not counting rogues or monks because they are skirmishers. They are built with multiple ways to get in and out of the frontline, and to have multiple ways to increase their surviablility in combat. Like a bonus action dodge, or a reaction half damage. Or evasion.
Druids have exactly zero abilities like this, and druids don't fight in a skirmish capacity. So, why would I compare skirmishers to a frontline build?
On par with what? That's the problem. Your think that you're entitled to have the same AC than the classes wit the best AC. This is not necessarily the case. Instead of thinking druids as default medium armour wearers that have unfairly been denied their medium armour AC think them as light armour wearers that have an awesome extra option to wear some magical medium armours. (And hide, but no one cares about that.)
On par with other melee frontliners?
Do you honestly think that 19 AC is the best AC in the game? Because that is a druid with a 14 dex, half-plate and a shield. A fighter or paladin can easily and trivially get 21 AC, before magic, before buffs, before any planning for a better AC. So, no, I'm not "entitled" and I'm not asking to have the same AC as the best of the best. Heck, I'm not even asking to be better than the wizard. A bladesinger wizard at level 2 (when they become a bladesinger) can pull off having an AC of 18. Put a few ASIs in them and that can end up as high as 23
before the use of the shield spell. Forge Clerics can have an AC of 22 by level 6, with no magical gear or buffs other than their class features.
And, just to reiterate. 19 is less than 21, 22, and 23. So, again, I'm not asking for the best AC in the game. I'm asking for a middle of the road AC that is at least decent for a front-line character.
So only the best possible magic items are acceptable? Yeah, sorry I don't see things that way.
You asked why I don't like Dragonscale, I told you. It is barely a fancy set of basic magic armor. It is disappointing for an iconic set of armor made from one of the most powerful creatures in the game. It would be like if you took the Ruby Rod and it was just a +1 damage and gave you back a single spell slot.
Oh right. But you seemed to have an issue with them having better AC than the druids. So that would fix that!
It would also fix it to remove all armor from the game! But, maybe instead of nerfing people so you can force druids to follow your aesthetic, you just loosen the grip a little and recognize that not everyone shares your taste in how a druid "should" be.
Everyone didn't buy medium armour from the shop. Rogues didn't, wizards didn't!
Though could have. It is only a single feat for Rogues to get Moderately Armored. Or maybe they are a dwarven wizard and can wear medium armor. Also, seven or eight of the classes can easily wear medium armor, so the vast majority of characters.
Rules often deal with mechanics...
And you shouldn't make a character's religious belief into rules.
This is more like wanting to player decide whether monks can use their martial arts in armour or not.
No, it isn't. Because Druids have no mechanics that stop working if they wear armor. Also, a Monk could decide to wear armor, they lose a lot of abilities, but they could make that choice.
A druid cannot make the choice, the choice is made for them.
So make it a level one spell.
I considered that. I wasn't sure about the impact. I haven't exactly had a lot of time to test this out and see if it works or not.
Ultimately I don't think that a class based game has reason to exist if classes are homogenised and can just be anything. But yeah, dropping the medium armour proficiency would be a decentish compromise, as it would still keep druid as default non-metal wearer. It is a very boring option though, and I would definitely prefer if they instead had some sort of thematic limitation on metal wearing like monks have with all armour and certain weapons.
And I don't think I want a game where the classes are so tightly restricted that I have little freedom to reskin them.
It is an interesting take, though I have to say that the AC scaling based on the spell slot level seems rather odd. Is there any other defensive or buff spells that work like that?
Ac spells? No. Buff spells, a few like Armor of Agathys and Aid. But I also think more spells should have better effects at higher levels. Upcasting is generally not better than casting a spell of the higher levels, so I don't see a problem with it generally.