Blizzard has discontinued things like HIT chance, Armor Penetration, and a whole slew of other things and just reclassified ALL items bearing those terms as something else. They've done it many, many times. "Item X is now Item Y". So...
What would happen if Blizzard decided to reduce armor type to the holy trinity: Plate, Leather, and Cloth?
I don't play a Mail Armor class, nor do I know much about it other than hearing for years that those players that do wear it are consistantly unimpressed with it's appearance. What would happen if Blizzard said "All classes that wore Mail now wear Leather. ALL Mail Armor in the game is being reclassified as Plate or Leather, depending on it's appearance"?
I know Leather and Plate classes would have more tier sets in their Appearance tabs to farm, but I'd be okay with that. Farming appearances is pretty much what many players do. And the classes that used to wear Mail Armor would gain access to all Leather gear, while a few players will have the novelty of having some unuseable gear pieces in their bags (common enough already. Returning players often log in to find now-unuseable gear in their bags). Also, less work for Blizzard when it comes to making gear for future expansions.
Some players will have unlocked appearances new players no longer can (ie. the mail pieces reclassified as Plate), but that's also common enough. As a transmogger, it's hella annoying to find the perfect piece, only to go to Wowhead and see the "This Item has been removed and is no longer available" message. But it's a reality I've accepted. Also called precedent.
I'm sure there are many other pros, and many cons. What are they?