I just don't understand this reasoning. Every expansion is temporary. Every zone, every raid, every dungeon, every quest, everything in a new expansion is temporary. It lasts until the expansion is over, and then you move on to the next one. Why can't this be the same for capital cities? By making new capital cities, the world only gets bigger and more beautiful.
I agree a lot of capital cities have been neglected, but it's no reason to never have new ones again.