Blizzard is doing what's necessary for consistency. You do not find mail pieces with STR on them past level 40. You do not find mail pieces with AGI on them before level 40. Yet, there are going to be mail wearing classes in Legion who need mail with AGI below level 40 and classes who will need plate items with str below level 40. This change HAS to happen for the sake of consistency. It's intended, and like I said, you will see new items added to fill the void this creates for lower level plate wearers. In some cases they are changing the stats on the item, other cases they are changing the armor type.
Does any of this make sense to you? They simply can't leave these items as-is. It's counter intuitive to the very design elements they are changing in Legion to make things more consistent as you level. You don't need to figure out why your armor class changes between level 39 and 40. You don't have to relearn which loots are for you at level 40. Starting at level 1, you just get plate for paladins and warriors, mail for hunters and shamans, leather for druids, monks, and rogues, cloth for mages, priests, and locks. That's it. It's really straight forward. With this new design, goblin mail leggins and thorbia's gauntlets no longer fit the players who are wearing them.
Does that suck a little? Sure, it never feels good to lose something... unless something better comes along to replace it. Which is what is happening in Legion. You should Embrace It. This will be good for the game.
Trust...