I only read this manhwa recently and I'm very late to the party, but it's so reassuring to find the discussion and to share the view with many. Guess that's what the forums are for.
This way of writing reminds me of Ichigo 100% (one of my youthful mistakes). Great art, started out with fated girl, ended with the bland one, but all so much more interesting compared to the dumb i-dont-get-who-can-love-this protagonist. By the end Sungji seems so much like a third wheel, and the Will side story is the highlight of why Sumin is so hated.
After chapter 50, like people said, I pretty much read for the boobs, and continued till the end for the boobs. Amen.