Our Roman Catholic friends seem to forget one of the primary reasons for establishing the canon of the NT.
So we would have a written record of the things believed by the early ekklesia to compare heretical teachings against.
That the canon exists, and for this reason, is evidence enough for sola scriptura.
The word canon means "rule". The Scriptures are the rule of the Christian faith for all those who want to know it. What Jesus taught, what the Prophets of the Lord were told to speak and write, what the Apostles and disciples of Jesus were inspired by the Holy Spirit to teach, record and disperse throughout the assemblies for the benefit of all believers that were born and who were yet to be born, is preserved for us in Holy Scripture. I simply cannot comprehend how
anyone could possibly think that the Scriptures must be subordinate to men or that its authority is subject to their judgement.
"But a most pernicious error widely prevails that Scripture has only so much weight as is conceded to it by the consent of the church. As if the eternal and inviolable truth of God depended upon the decision of men!"- John Calvin
"The Scriptures obtain full authority among believers only when men regard them as having sprung from heaven, as if there the living words of God were heard."- John Calvin
Semper reformanda secundum verbum dei - The church reformed, and always being reformed according to the word of God.