What you and Rome are ignoring is the natural processes by which the Old and New Testament was completed. In both cases oral teaching preceding putting it into writing.
In the case of the Old Testament the Jews, just like Rome, tried to perpetuate oral teaching preserved as tradition and thus we read of "the traditions of the elders." Jesus repudiated the oral traditions. Jesus NEVER ONCE quoted them or referred to them as a source of authority for anything he taught. However, the Jews used the exact same reasoning and arguments that the traditions of the elders were equally sacred as scriptures just as Rome does:
"ORAL LAW - According to the rabbinical interpretation of Ex. xxxiv. 27, the words indicate that besides the written law——God gave orally to Moses other laws and maxims, as well as verbal explanations of the written law, enjoining him not to record these teachings, but to deliver them to the people by word of mouth (Giṭ. 60b; Yer. Meg. iv. 74a; comp. also IV Ezra [II Esdras] xiv.). The expression "Torah shebe-'al peh" denotes, therefore, "the law indicated in the word ' 'al peh,'" and hence only the law which was given to Moses orally. But even disregarding that Talmudic interpretation, the expression is equivalent to the Torah, which was given orally (), not in writing. Compare (Soṭah vii. 7), used of a recitation of the Biblical text by rote. In a wider sense, however, "Torah shebe-'al peh" includes all the interpretations and conclusions which the scribes deduced from the written Torah, as well as the regulations instituted by them (comp. Yoma 28a, b and Rashi ad loc.), and therefore comprises the entire traditional teaching contained in the Mishnah, the Tosefta, and the halakic midrashim, since these were taught only orally and were not committed to writing. In later haggadic statements, however, the complete body of rabbinical doctrine is said to have been revealed to Moses on Sinai; so that R. Joshua b. Levi declared (Yer. Peah ii. 17a) that all the rabbinical teachings, even those which the scholars found and promulgated later, were given to Moses on the mountain (comp. also Ber. 5a). - Online Jewish Encyclopedia "Oral Law"
Not only did Christ reject the "oral traditions" but so did the early churches. Yet, Rome takes up the very same line of argument as did the Jews in regard to oral tradition by regarding it equally "sacred" as did the Jews with the "traditions of the elders." Indeed, the "traditions of the elders" became the standard authority for interpreting scriptures JUST LIKE ROME's use of "sacred Tradition" does.
There is no more basis to accept Rome's "sacred traditions" as there was for Christ to accept Israel's "sacred traditions."
Isaiah 8:20 denies that right to any "traditions" not committed in writing as scriptures.