During Aang's life, Yangchen told him that he'd need to "sacrifice his own spiritual needs and do whatever it takes to protect the world." Throughout this novel, we see Yangchen's philosophy at play in her actions and decision-making. Not only did she neglect her own spiritual needs, she even took on a highly political role in mortal affairs, even engaging in spycraft, manipulation, backroom dealing, and even used Zaheer's suffocation technique (albeit right up to the line of lethality).
In the novel, we see Yangchen even took this to extremes at times; she'd made guards out of Air Nomad monks to keep the combustionbenders prisoner, she made deals with spirits that often favored mortals (and were still broken by them), among other things. In the end, she neglected her spiritual duties to the World to such an extent that her successors all the way up to Aang (an entire elemental cycle) were STILL cleaning up that mess. She didn't just neglect her own spiritual needs, she neglected the World's spiritual needs.
Jetsun is implied to be trapped in the Fog of Lost Souls. Many people never return from it. And even if Yangchen does save her, how many people have returned to the Physical World, only to find their bodies have long passed, and are already decomposing?
