Hello,
You said:

“There is no reason to believe that the New Testament writers were speaking of some infallible “unwritten Tradition” that was separate from the message of the New Testament and that was to be passed on through an unbroken succession of bishops throughout the ages.”

That may be true, but the problem is also that there isn’t any solid evidence to suggest that all the Apostles meant to say was written in Scripture. If Paul told the Thessalonian Christians to hold fast to what was written and also preached by word of mouth to them, that could mean what you say it means, or it could mean that there are unwritten traditions today that we are meant to follow. The idea that all apostolic tradition that we were meant to follow was written in Scripture is an assumption – and, perhaps, not a very good one either, since two very ancient churches and many church fathers would disagree with you.