Average ACs by my estimation; not counting single round boosts to AC or items, without intentional optimization for AC:

Barbarian: Unarmored: 15-21; Armored: 17
Bard: Lore: 14-15, Valor: 18-19
Cleric: 18-20
Druid: 16-17; if Metal Armor is Allowed: 19
Fighter: 17-21
Monk: 15-20
Paladin: 17-21
Ranger: 17-19
Rogue: 17
Sorcerer: Dragon: 15-16, Others; 14-15
Warlock: 14-15
Wizard: 15-16

So access to metal armor is a 3 point boost to AC (or a 2 point plus freeing up stat points); which isn't world rocking but is substantial. Druids already sit above all the Arcane casters in terms of sustainable AC (barring Valor Bards); is there a real need to push them up above all but heavy armor users? I understand it won't break the game mechanically if you do, but... it doesn't break the game mechanically if you don't let them either.