Why? Premium members might have some insight as to what it would take for them to see value in a Legendary Sub. I would sub up again if I had access to all content scaled for solo play. I also think you're under valuing vanity items as a unique member reward. Removing things from premium is just as valid a possibility as adding things to the sub. It's smarter to let Legendary members keep somethings they already have as a members, by taking those things away from premiums. Access to free vanity items is one of those things. Do you want free access to all earnable vanity items in the game? It's going to be difficult to maintain the longevity of new content if the Membership rewards eliminate too much grind. Vanity items fill the gap.