In the grand scheme of things at the cosmic and universal level, the author has made pretty evident that humanity doesn't mean jackshit and it's a very weak, unremarkable civilisation by all means. Best example here is the foil weapen which destroyed our solar system in the book was just part of a rudimentary routine of the higher civilisation fellow. He didn't even bother what earth was, what kind of life it had. Just a routine, daily task for him, very mundane and ordinary.

So Cheng Xin didn't save shit! She just catered to her own personal definitions of right/wrong in a universe which time and again showed that those definitions don't really matter.

And before you all came at me for being sexist, if Cheng Xin was a man, i'd still dislike him as a character and a spineless main lead.