You're contradicting yourself now.
You mentioned systems before and now you're advocating for permanancy. Systems are the antithesis of permanent content. In fact, systems that are tied into player power - artifacts, AZ armor or covenants - are actually Blizzard's shortcut to avoid having to deal with the base talent trees directly. So you see, classes have actually been designed around a temporary focused philosophy.
Moreover an entire world revamp is unpractical. What they can do is do what they did in Cata. Focus on a few zones - Uldum, Twilight Highlands - as the new content and possibly upgrade a few more here and there. Then, throughout the course of the expansion, upgrade some more.