In the Invincible comics, Rex survives. The character's fate takes different turns, but death isn't one of them. The storyline keeps him involved and active throughout.
Allen doesn't die. The creators of Invincible comics have kept him alive to drive the story forward and engage the readers with his ongoing adventures.
Well, in some comics, the invincible characters might die if it serves a greater narrative purpose or to shock the readers. But in others, they might always remain unbeatable. It really varies.