of course coral <3 sangsang for you. so... i dont think my followers know that this is like one of my favorite limbus ships ever because i just love the themes of self love and self realization. sang yi is a really interesting character to me on his own and i always hope he comes back at some point... because his reationship of someone who uses his power over yi sang to keep him trapped and yet eventually changes his mind is so fascinating. imo he never wanted to manipulate yi sang or he wouldnt have felt any empathy for him when he realized the bad position his advice had put yi sang in. my read of sang yi is of a victim (we dont know his exact circumstances, but he wears t corp clothes in the first scene we see him in which implies at least he gave into the same authoritarian pressure yi sang was under in his time there) using a flawed coping method trying to help another victim with that same method and instead making things worse for him. the role of yeonshim is split between gubo, the actual perpetrator physically keeping yi sang trapped, and sang yi, who is both the external opinion of someone who wants to help yi sang AND yi sang's own voice within himself that reassures him that he can only be safe if he stops striving for anything and just stays where nothing can hurt him. you can see it allegorically as a fictional ideal of just forgetting about everything you used to care for and achieving peace in that silence.
but sang yi's life isn't perfect, he's lost most of what made himself and what he used to care for just for that safety he desired.
that's why i think the realization that yi sang didn't value himself at all was a hard hit for sang yi, and he came back to tell yi sang that he does have the option to try something else, and sang yi can't guarantee how it'll go because it was what he always tried to avoid but yi sang has the chance in front of him to leave and face the unknown. he has a future out there. that way yi sang sees that sang yi's wings were always a reflection of his own and that everything he loved about sang yi is also within himself.