As a healer main, and male roe. Robes have almost never looked good on me. The purgatory top was one of the first real healer tops that I felt was "yeah this is nice".
As pointed out, sure a black mage in plate would look weird, but we already have armors like that for them PLUS if someone wants to be an armored mage why not?
Clerics exist, they're just a paladin that is also a white mage. What about the "rookie adventurer" trope. Where the rookie buys the finest armor that doesn't match his class at all. If it's not impossible then why not give us the freedom? no worse than jumping into a dungeon with a meme roe wearing the banana hammock and a chocobo mask.