The perfect suusano is only attainable through rinegan.The perfect suusano is the only thing capable of standing up against a tailed beast this is why it is pretty obvious sasuke is going to get the rinnegan he needs it to be able to fight on terms with kurama mode naruto
i thought the perfect susanoo was through the eternal mengekyou not the rinnegan. the rinnegan has its own abilities not susanoo.
the rinnegan can be obtained through evolution of the eternal mengekyou that's why people think he will get it.