They fooled me at first with the Nezuko flashback flagging her death, and I was impressed that they could possibly kill her off... Of course it was a fake out. The random retconned powerup was fairly eye rolling and ruined a good chance for a meaningful character death, but what annoys me most in hindsight is that this immunity would have been so easy to foreshadow at all and not make this so unsatisfying. Even a small mention of her blood mutating by doctor demon would have been enough. This reeks of an unplanned story change. I've heard the ending of the manga was rushed, and maybe this development was the beginning of it.

Either way, this does serve as a good paradigm shift and thrust some more urgency on the story, now that both sides have a defined goal.

Overall though, this was easily the weakest arc yet, and hopefully a low point for the entire series, because it really felt like it was just going through the motions for most of it. I wanna be hyped for Kimetsu again.