27) If sola Scriptura is so solid and biblically based, why has there never been a full treatise written in its defense since the phrase was coined in the Reformation?

As sola Scriptura is so solid and biblically based, and elementary, there was never a need for a full treatise written in its defense.

A treatise is a formal, lengthy, systematic exposition or argument in writing including a methodical discussion of the facts and principles involved and conclusions reached.

No need for a treatise when it is written:

KJV Deuteronomy 4:2 Ye shall not add unto the word which I command you, neither shall ye diminish ought from it, that ye may keep the commandments of the LORD your God which I command you.

The Word of God has supreme authority that is not to be diminished by the addition of the word of man, or in our technical jargon, sola scriptura now that the Word of God exists undiminished by the word of man only in our Scriptures.

But lest it be supposed man has not found the need to belabor the point:

The authority of the word of God: A treatise on the criterion of Christian faith and life by William Carl Heyer

Holy Scripture: The Ground and Pillar of Our Faith, Volume 1 - A Biblical Defense of the Reformation Principle of Sola Scriptura by David T. King

In every Systematic Theology, every doctrinal statement, they "Treat" the principle of sola scriptura, "Treat" is from whence the word "Treatise" came.

So sola scriptura is treated so often I wouldn't dream of wasting time listing the titles. Almost every Bible Christian book about Scripture, the faith, the church, etc., will treat the subject. Take your pick.

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The times of this ignorance God winked at; but now commandeth all men every where to repent:

Behold, I stand at the door, and knock: if any man hear my voice, and open the door, I will come in to him, and will sup with him, and he with me.-kjv