To add to all the other "What if Legendaries..." threads. What if they were purely cosmetic, like Green Fire or the Visage of Tarecgosa 'mount'?
They wouldn't be considered mandatory, they wouldn't require instances be balanced around their presumed presence and so their acquisition could be based more on skill*, dedication** and desire rather than a perceived "need". That would ensure those who really want them get them to keep their prestige, and that prestige would have lasting value beyond the tier from which is was earnt. To me, the present perceived need is the cause of what makes them so prevalent and so unlegendary.
*Skill allows something to be obtained through ability.
** Dedication allows something to be obtained almost regardless of ability but is made up for in sheer temporal investment.
These are both things that are already rewarded in the acquisition of Legendaries - Skill usually allows you to get them sooner than dedication alone, which is a fine model as far as I am concerned.