From the developer interviews before launch:
In other words, they will continue to add new traits, or new ranks of the hidden traits, probably at the same time as new raid tiers.Artifacts are something that will continue to grow and evolve so that Artifact Power matters for the entirety of Legion.
After doing one week of world quests, how do you feel about dedicating the next 2 years of your life (expected expansion lifetime) doing them every single day? Of course, you don't HAVE to do them. If you're fine with not being able to compete either in pvp or raiding.