Horizontal progression is just an illusion of choice The whole static mentality is a community issue and nothing to do with the design of the game itself. All you get with horizontal progression all you'll see is the raid gear all of it being BiS for all classes and the set below it that is slightly worse which every one will choose and the other 3 or 4 sets that are trash and people will only do the content it comes from if they like how it looks.
