I understand that is your premise. What I don't understand is how you conclude the text supports that premise.
Could you demonstrate, please?
The only conclusive statement I can draw from the text is that the man of God CAN NOT be "complete" without knowing the Scriptures. That is a very different proposition than claiming the Scriptures alone "complete" him.
Paul declares four areas that properly applied in the persons life are adequate "so that..." the person can be complete, perfect, equipped thoroughly for every good work. He says that the scriptures provides those four things. There is nothing, I suppose, that says those things could not come from another source, but scripture of which Paul writes is a sufficient God-breathed package for the making of sanctified disciples of Christ.
We're solving a puzzle. We have a box with pieces. When the pieces are put together, we will have a beautiful finished product. The manufacturer guarantees that the pieces are all there.
Along comes a dear friend who loves to do puzzles together with us. He has a box with pieces in it. The picture on the box certainly looks similar. But his box does not have the same manufacturer's guarantee. And his puzzle has 500 pieces.
He insists on mixing his pieces with those from our box, arguing that we will still get the same picture. But now instead of a 500-piece puzzle, we have 1000 pieces.
Along the way, our friend, who is a real puzzle buff, finds the same picture in another box, and we repeat the process, but now we are trying to assemble 1500 pieces, when the original 500 would have gotten the beautiful picture we want.
Of course, the Catholic Christian says that the second and third boxes do have the manufacturer's guarantee. But even if it were so, a simple puzzle has grown unnecessarily complex.
Paul said that the scriptures were able to make one wise unto salvation. From that point on, my premise becomes that scripture is sufficient to salvation when applied.