I saw a theory somewhere (I don't remember) that said people in the Pure World can't be brought back to life by RT, only the ones who are in the gap between this world and the Pure World (like Kakashi).
sage can't be revived because its been a while when he had gone.
rinne tensei can only revive people who died withing the jutsu time range.
sage died ages ago. even madara can't revive him.
If the the Rinnegan could not revive people who have been dead for ages, then was Madara's plan to be revived via Rinnegan a failure? Doubt it, I'm sure the Rinnegan can revive anyone no matter when they died.
Probably can't find his body or DNA. They need something to focus the chakra on. Also, there is a theory that someone had, that I do not necessarily believe, that the SO6P is still alive and observing everything haha