But that assertion is not in the scripture.
Wow. Read your Bible, annalex.
Try 2 Timothy 3:15-17.
That passage does not say that the scripture teaches us all we need. It says that study of the scripture (including the books you guys threw out) produces a perfect man of God. But it does not say that the study of the scripture alone does that; in fact, the preceding verse says the opposite: "continue thou in those things which thou hast learned, and which have been committed to thee: knowing of whom thou hast learned them". In other words, learn the traditioon, then finish with the scripture.
Read the Bible with love and attention and you will become Catholic and abandon the Protestant nonsense.