It was a place without Death, and Death clearly appeared to instill death in it. However, she also seems to abandon Thanos. It's entirely possible that having killed the Multi-Angled Ones and Mar-Vell, Death was satisfied enough to return home. When Death leaves, does the universe return to its undying state? It's possible, and that's what we've been shown.

As for the Cancerverse not collapsing, do we have anyone with the know-how say that it's collapsing? The portal to the main universe is, certainly, but does anyone other than Quill say the universe itself is dying? He's certainly fallible.

Mind you, I don't find the story greatly satisfying either. The Revengers' Cap's shield changes shape for no reason, Nova himself receives a grave injury needlessly, and the use of the Cosmic Cube doesn't line up with what we were told in Thanos Imperative. Hopefully the final act will be stronger.