Why dont we just give a certain time limit on getting kills on heroic progress with achievement on EACH boss with cosmetic rewards for achieving that challenge before nerfing the boss for the rest of the player base to finish end game content. Say 4 to 5 months to finish a heroic raid content then nerf and remove those achievements??
Think it kinda work that way already, at least with the "Cutting Edge" achievement you get from killing the endbosses of each raid. When 5.2 hit these will dissapear.