The letters were preserved...God said he would preserve his words forever...Thus we have our bible..
2Th_2:15 Therefore, brethren, stand fast, and hold the traditions which ye have been taught, whether by word, or our epistle.
You have neglected to notice that Paul didn't sanction anyone to continue to create traditions that would become church dogma or doctrine...
You have also neglected to acknowledge the verse does NOT say, 'hold the traditions you have been taught by word which are not contained in the written scriptures'...
What it does say is, 'hold the traditions you have been taught by word of mouth, OR hold the traditions you have been taught by the written scriptures'...
Hold fast to the traditions whether taught by word of mouth or written scriptures...
Doesn't occur to you they may be the same thing, eh???
The verse says you can chose either, the oral word or the written scriptures...Doesn't that seem a little odd, unless they are the same thing...
And since none of those supposed Catholic traditions by word of mouth survived history the poor folks back then had to resort to the written epistles and only got part of the truth???
However, God clears it all up with his instructions to Paul when it comes to those Catholic traditions:
2Th 3:14 And if any man obey not our word by this epistle, note that man, and have no company with him, that he may be ashamed.
2Th 3:6 Now we command you, brethren, in the name of our Lord Jesus Christ, that ye withdraw yourselves from every brother that walketh disorderly, and not after the tradition which he received of us.
Obviously that 'tradition' was contained in the scriptures, or was the scriptures...
"You remember that thing I was discussing with you over dinner?
"Forget it; for the true facts are..."