I ask you again, where does it say something has to be written down?
We are not to exceed what is written.
Pretty simple there.
You did not answer my question.
If I tell you "Don't go beyond 60kph when driving" and you read "Don't go beyond 60kph when driving" then the spoken word has not exceeded the written word, and still doesn't say "It must be written"
Here is what we have. We have what is written. Then we have what some call tradtion. So in comparison it would be like this. On the speed limit it says that the speed limit is 60 MPH. But the tradtion of the road speed is 70 MPH. So instead of following the written someone follows the tradtion and therefore is breaking what is written. But because someone said that actually it says 60 people that travel that road everyday do not go 60 but 70 so therefore we have tradtion breaking and becoming ruler over what is written. Not good. 
And, in point of fact, if this were the case then Paul, when he went from town to town preaching could only have done so by writing everything down then and there.
What He preached would have been the very same as what He had written. So now that Paul is no longer among believers we have now to go with what it was that He wrote for He is no longer around to speak orally to us. We only have His written. For the tradtions Paul speaks of is from Him and not from hand me down of what someone thought he said.
Romans, Philemon, Hebrews, & Colossians had not yet been written down. So for you were the 'lessons' not truly from God until Paul wrote them down?