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Are you saying that after the NT was completed, the 12 apostles and other disciples of the NT did not preach the Gospel of Christ to all the nations? According to the NT, they did. So that is spoken oral apostolic tradition. If there is no need for tradition, then 2 Thessalonians 2:15 is false. If 2 Thessalonians 2:15 is false, all of St. Paul could be false. Why stop there? Why not only doubt the rest of the NT, but also the OT as well. The idea of no tradition added to the NT Scripture falsifies and makes void 2 Thess. 2:15.
You are assuming tradition was not needed after the Scripture was completed. You are assuming Scripture is complete without the Church that wrote the Scripture. 1 Tim. 3:15. The Church gives the true meaning of Scripture. Without the Church, there is only private interpretations of the OT and NT Scriptures.
Tradition is needed in places where Scripture is not clear.
Tradition is needed in all places, for Scripture is part of Tradition. Scripture is inspired written Tradition. There is inspired oral Tradition as well.
All of them: Scripture and Tradition, and the Church, come from the same Holy Spirit.