It’s more a common sense conclusion. Oral tradition isn’t going to survive that long with much accuracy. Now if the tradition is preserved by the holy spirit that’s another matter, but I also don’t see evidence for that conclusion. If there is, please show me.
How did the account of salvation history get from Adam to Moses?
You sure don’t seem to have much confidence in God being able to protect His Word.
1 Thess 2:13
13 We also constantly give thanks to God for this, that when you received the word of God that you heard from us, you accepted it not as a human word but as what it really is,
God’s word, which is also at work in you believers.
God placed His Word in the Church by the Apostles. He is able to guard and to keep it there/ It will remain there, infallible, until He comes again.
Isa 55:11
so shall my word be that goes out from my mouth;
it shall not return to me empty,
but it shall accomplish that which I purpose,
and succeed in the thing for which I sent it.
Why do you reject the teaching of Jesus on this matter?
Matt 24:35
Heaven and earth will pass away, but my words will not pass away.
God’s purpose in giving His Word to the Church was to purify her. When she is joined to the bridegroom at the end of the age, His purpose will be completed.
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There wasn't a gap between the existence of the bible and the death of the last apostle. It doesn't make sense for the writings, which we know for certain were in existence, to have been ignored for 200-400 years. The church, gifted with the holy spirit, would have the ability to discern them right away. Why wouldn't it?
So you can trust the HS to lead the church into discerning which writings are Holy Scipture, ,but you can’t trust the HS to maintain the Word of God in the believers?