They're not bringing back Poro King. They're just looking to add a similar summoner spell to the game to give melee champions a better means of engaging in ARAM. The game mode will still be ARAM.
And ARAM isn't going to die off. URF would. URF did, as with all of the featured modes. People play it a few times right away for the novelty, and then the novelty quickly wears off and participation in the mode falls away. ARAM has held strong as a popular mode for over 5 years already. Sure, Howling Abyss and the fully-supported ARAM game mode are much newer than that. But people were playing custom game ARAMs for years before HA came out. The custom game lobby was always flooded with ARAM games being set up.