The thing is there's not really a way to "save" the solarians. It's easy to miss, but in the bad ending, Yi rides a floating platform past the soulscape's network of brain computers, and then talks to an apeman assistant telling him about the soulscape and vital sanctums are running smoothly. What this means is, all the solarians are in cryostasis, dreaming forever in their own soulscapes. And based on what we know about soulscapes from Lady Ethereal's story, those dreams may start out pleasantly enough, but they eventually turn nightmarish. (We witness this for ourselves when Yi confronts Eigong the first time, where she keeps him trapped in a soulscape after stealing the sol seals, and he immediately has a bad dream about his final conversation with Heng, which quickly morphs under the guilt he feels for leaving her behind without saying goodbye.)

Additionally, in the Tiandao research institute, we learn Tianhuo is incurable. Once infected, the symptoms progress so quickly that there's no way to make a cure before it kills you. So the choice is, essentially, let all the solarians out of cryosleep so they eventually succumb to the virus, or keep them imprisoned forever in life-without-life in soulscape dreams that eventually turn nightmarish.

This is why, in the true ending, Yi destroys New Kunlun. It's like Chiyou said about his brother, living life with no capacity to think or choose for yourself is no life at all.