If they make Artifact Weapon Appearances unavailable to unlock after Legion
I already know some people are going to argue the prestigious argument and what not but I think you need to consider the benefits in the long run. Soloing raids/dungeons from previous expansions is hugely popular and is an incentive for people to keep playing be it trying to get that mount or mog item! If Blizzard remove Artifact Appearances that severely reduces the incentive for people to spend time soloing Mythics and Raids in Legion in the future considerably. I honestly think removing them will be a wasted opportunity and also demonstrate that the amount of work going into them was wasted if only a handful have them