IDK. I mean, it's not like they haven't found ways to keep YED current in the canon, you know? It would be no different to me to bring him back, or Ruby, or Alistair than it would be to bring John or Mary back. They've each in their own way had just as much of an impact on the story and on Sam and Dean.
Though I suppose there's a difference to me between bringing YED back or Alistair, Lilith, or Ruby. YED was killed with the Colt and I like the idea that death by Colt is a true, soul death. But there's nothing to say that Sam's powers or the knife causes a death of the soul, so it wouldn't bother me nearly as much if Alistair or Ruby, or even Casey came back, especially if they couldn't leave Purgatory for whatever reason and so had a limited role to play.
Re: John's and Mary's absence from heaven (small spoilers)
IDK. I mean, it's not like they haven't found ways to keep YED current in the canon, you know? It would be no different to me to bring him back, or Ruby, or Alistair than it would be to bring John or Mary back. They've each in their own way had just as much of an impact on the story and on Sam and Dean.
Though I suppose there's a difference to me between bringing YED back or Alistair, Lilith, or Ruby. YED was killed with the Colt and I like the idea that death by Colt is a true, soul death. But there's nothing to say that Sam's powers or the knife causes a death of the soul, so it wouldn't bother me nearly as much if Alistair or Ruby, or even Casey came back, especially if they couldn't leave Purgatory for whatever reason and so had a limited role to play.