They're not the same thing. rsync is unidirectional, and only between two endpoints. One side is always overwritten by the other side. syncthing is bidirection

They're not the same thing. rsync is unidirectional, and only between two endpoints. One side is always overwritten by the other side. syncthing is bidirectional, syncing changes in either direction, and supports syncing any number of clients, not just two. Watching and syncing immediately - yes, that's what syncthing does. Why reinvent it with something worse?