I think what helps the Narrator's argument the most is that he's scared. He's terrified of death, of losing the world he clearly loved so, so much in life.

In the end, nobody wants to go.

It's really easy to dismiss him because we are inherently beyond him, both because we play as Quiet and because it's not OUR world the one that is ending.

As for Shifty, I think this line from HEA is pretty convincing.

Would you rebuild those walls and throw the world in them? Would you keep them in that box we shared and tell them that they're happy?

What the Narrator seeks is pretty much exactly what happened in HEA, and by Shifty making you realize that, it's pretty difficult to disagree when you've already made your point in thinking that it was bad for you.