Always left out of the discussion is mention that the hypothetical Council of Jamnia determining canon is NOT some hoary tradition. It was made up about 140 years ago.
I myself, brought up the doubt, here on this thread. Why post to me "it is always left out"?
Made up some 140 years ago? Well, ok, maybe, but then again there have been enough scraps of "activity" (just that I know of!) traced to the first/second century, related to objection from some Jewish quarters concerning their disagreement.
Some of that I think it safe to assume, was the stuff the "Jamnia" council hypothesis was founded upon. There is evidence there...enough to give us a sense something, some activity akin to it, transpired.
There was a network of schools or colleges that the Romans destroyed along with Temple in Jerusalem. Would we expect that they were able to pry out of the mind and conscience of the Hebrews which studied there, knowledge of such primary importance to them, as to what their Scripture was, and was not?
Like I said --- go to the Jewish scholars, and ask THEM why they hold the "canon" which they do presently.
They claim to be carrying it forward unchanged for many more centuries than the Catholic Church does, for what they carry.
At issue is not what writings which can be found and translated (and have been, but not uniformly) but which particular ones were considered canonical at the time of Christ.
What did the Sanhedrin hold to be canon? That is the question.