A pipe-dream is "an unattainable hope or scheme", according to Google. Weapons always break in The Last of Us, so an unbreakable weapon is technically unattainable (for Joel). A cryin' shame, too, 'cause Ellie's got that invincible knife right in her pocket. Think of all the throats you could slit with that thing.
Oh.... then I had it all wrong. I thought, pipe, as in the one you smoke. A pipe dream, I guess, would be something you would see while you were high? Man, am I an idiot. As to Ellie's knife, she didn't craft it out of scraps. It's a sturdy factory-made knife. Joel's shit always breaks because he's not exactly an engineer.