From the very beginning Seonwo was not supposed to alter the original plot, he was a character that is meant to be discarded from the original plot itself. For him to get that skill(sacrifice) that means he was meant to be gone before even getting the progress of the story itself. He was a side character that isn't meant to be side by side with the protagonist and other main characters. His sole purpose was to be gone.
From the very beginning Seonwo was not supposed to alter the original plot, he was a character that is meant to be discarded from the original plot itself. For him to get that skill(sacrifice) that means he was meant to be gone before even getting the progress of the story itself. He was a side character that isn't meant to be side by side with the protagonist and other main characters. His sole purpose was to be gone.