Why do 21st century Protestants still not include the deuterocanonical books in the canon? Given the Protestant motto semper reformanda and the doctrine of sola scriptura which in essence strives to purify Christian teaching to be based only on the Word of God interpreted through the lens of the 12 apostles, what is preventing Protestants today from including the Catholic deuterocanonical books back into the canon when it can be shown:


How the NT writers did make allusions to those books
How recent research showed how the NT writers relied on the Septuagint's version of the underlying Hebrew OT vs. the Masoretic text
Appeal to what Judaism itself consider as canon (i.e. 39 books) is irrelevant because Christianity should have its own hermeneutic, especially when the Septuagint text could be shown as less tainted by errors now.
How the two highly voted answers for the question "Why were Deuterocanonical books rejected in the Reformation?" fails to show a good reason for their exclusion from the Protestant canon because none of the reasons mentioned are derived from sola scriptura:


Luther and Calvin were motivated to either diminish or eliminate the deuterocanonical books to exclude scriptural support for Catholic doctrines
Appeal to St. Jerome's concern of the reliability of Septuagint has since been reversed after the discovery of the dead sea scrolls (see 2nd point above)
Synod decision post Luther &amp; Calvin should in principle be re-evaluated when the apostles can be shown to rely on Septuagint more than what was known 400-500 years ago



500 years have passed since the violent and emotional schism.  Both the Protestant and Catholic sides, especially in the academia, have since understood each other much better and became much less polemical.  In the past 100 years the trend for both sides is to dig deeper into each NT author's 2nd temple Judaism worldview to better inform our understanding of the NT books, which is clearly also in the spirit of sola scriptura.

Of course, one obvious reason is because the Catholic church had developed doctrines unacceptable to Protestants based on those books, like Purgatory, indulgences, etc.  But if Protestants use proper hermeneutic to guard against "overzealous" interpretation of those deuterocanonical books (such as by controlling interpretation to what only the 12 apostles taught in the 27 NT books), why not include them in the canon?

What makes this question different from previous questions such as this one is the 21st century context because in principle Protestants are not bound by past decisions (per semper reformanda principle), but bound only by what the NT authors taught (sola scriptura).

Therefore, my question asks for a 21st century argument for rejection based on those 2 principles (semper reformanda and sola scriptura) which should not include polemics and outdated Masoretic text / St. Jerome argument as in the answers of the previous question.

Another way of stating the question (positively): given 1) fresh discovery in the 20th century into Jesus and the 1st generation apostles's OT background (such as the texts they use) and 2) given Protestants's commitment to the semper reformanda and sola scriptura principles, what criteria does it take for Protestants to include back the deuterocanonical books that they rejected 500 years ago in order to fulfill their ad fontes ideal of being faithful to the original revelation?
Because Protestants do not believe that the Holy Spirit and Jesus Christ have borne witness to them that those texts are inspired scripture.

The Westminster Confession (1646) has a fascinating paragraph where it lists off many of the characteristics that set scripture apart from other writings, before dismissing them all to say that the ultimate reason we are persuaded that the scriptures are inspired is the inward work of the Holy Spirit and Christ in the lives of believers, witnessing to us.


  WCF 1.5: We may be moved and induced by the testimony of the Church to an high and reverent esteem of the holy Scripture;
   and the heavenliness of the matter,
   the efficacy of the doctrine,
   the majesty of the style,
   the consent of all the parts,
   the scope of the whole (which is to give all glory to God),
   the full discovery it makes of the only way of man's salvation,
   the many other incomparable excellencies,
   and the entire perfection thereof,
  are arguments whereby it doth abundantly evidence itself to be the Word of God;
  yet, notwithstanding,
  our full persuasion and assurance of the infallible truth and divine authority thereof,
   is from the inward work of the Holy Spirit,
   bearing witness by and with the Word in our hearts.


The same idea is present in the Belgic Confession (1559):


  Article 5: We receive all these books and these only as holy and canonical,
  for the regulating, founding, and establishing of our faith.
  
  And we believe without a doubt all things contained in them—
  not so much because the church receives and approves them as such
  but above all because the Holy Spirit testifies in our hearts that they are from God,
  and also because they prove themselves to be from God.
  
  For even the blind themselves are able to see that the things predicted in them do happen.


Now of course as sinful humans we can ignore the work of the Spirit and Christ. But let's consider another example of an issue where I think the Spirit is surely trying to convince the Church about the truth of a matter: infant baptism. For centuries Protestantism has been divided over whether or not infant baptism is a good practice. Individual Christians sometimes switch from one camp to the other, in part I'm sure because of the work of God. It is the Spirit who helps us interpret the scriptures (WCF 1.6) and yet we have not come to an agreement over what the scriptures teach about baptism. Is that because we hold too tightly to our traditions to hear what the Spirit is saying? Perhaps for some people, but for others it appears that, if they are switching camp from falsehood to truth (whichever direction that really is), they are heeding the voice of the Spirit.

What about for the canon? There is no major group of Protestants who believe the dueterocanon is inspired, there is not even a minor group. I can't even think of any notable individuals who have said this. Over five centuries have hundreds of millions, possible billions, of Protestants all tuned out the Spirit's witness to the authority and inspiration of those texts? That does not seem plausible to me. We know that the Spirit has been a witness to the authority of the scriptures because we Protestants fully recognise James as scripture despite our founder Luther's reticence to do so. Instead I must conclude that the Spirit has not been witnessing because those texts are not inspired.