The-Magic-Sword
Small Ball Archmage
Your druid PC wearing metal armor? Then obviously they are not a druid. How do I know? Because the rules state clearly that druids won't wear metal armor. Period, end of story. If the PC can do anything the player says they can do, then my first level fighter can cast wish. While the DM doesn't make the rules for the PCs, the PHB and associated books do and they are clear. Want to run it differently in your home campaign? Feel free.
The DM enforces the rules, the players don't.
But you forget, we're playing an exception based rules system- if Druids as a general rule "will not wear metal armor" or in other words, refuse to do so (which is the intent of those words, as described by Sage Advice) then Carlos, The Druid who has, as a portion of their personality, that they are willing to wear metal armor, becomes an exception to the general rule of "Druids will not wear metal armor" this functions because the rule makes a general assertion about the personality of druids, but the player determines the specific personality of their character, who is a specific instance of a druid's personality. Nothing else in the game invalidates specific exceptions using general rules the way you describe.