Who the hell did you think she was!?!?

[OpMegs]

Simon pretty much states that Nia forbade him from breaking the rules of the universe just to preserve her own life.
Because if he did that, it would start an inevitable road of bringing EVERYONE back from the dead, and hasten the "Spiral Nemesis" where the entire universe is crammed with spiral beings until their mass collapses everything.

Yeah, but, she wasn't dead yet!  Bringing her back after she died would have been one thing, keeping her alive is something else entirely. 

I generally just cut off my memory of the series end at the wedding, before Nia faded away. You don't really lose anything, and we get to keep Nia, so I say it's a fair trade.
---
"Oh, silver blade, forged in the depths of the beyond. Heed my summons and purge those who stand in my way. Lay
waste."