We have datamined a new cutscene leading into the Vault of the Incarnates story, explaining why we enter the raid. Story spoilers.
In this cutscene, Alexstrasza goes to recharge the oathstone to fully regain the powers of the Aspects–but is puzzled why this fails. Until Raszageth shows up, proclaiming that the Aspects proved themselves unworthy of these powers on their own and now a new age is here–the Age of the Incarnates.