Jo has already pointed to the fact that this is not so. The book of isaih is the only complete book that has been recovered. You can't base the veracity of a whole canon on one book's integrity.
Besides, it's not am matter if the Masoretic Text is 'correct' or not but the fact that DSS do differ form it and do agree with the Septuagint, thereby shattering the fairytale that there was 'one Jewish canon.'
The fact that it agrees with LXX in some parts shows that the so-called "Hebrew" Bible (the canon of the Palestinian Pharisee sect, which everyone assumed to be the one and only correct version of the Old Testament is not what it was made to be.
The fact that the Apostles overwhelmingly use the Septuagint (over 90% of the time) shows that they considered it Scripture. Yet, it is not good enough for Protestants! Rather they use the canon of the Christ-denying Jewish sect.
The KJV "agrees" with the Paletisnian canon because that's the basis for its OT. No surprise there. The rest is based on the Textus Receptus, a "Greek" source retrotranslated into Greek from a Latin translation!
Add alittle Protestant flavoring that reeks in the KJV and you have a great source, indeed! Even the authors admit it was not inspired and list there are known lists of hundreds of errors in the KJV.
The DSS revealed that, yes, the Masoretic Text's Isaiah is faithfully preserved, and no, the Palestinian (Pharisaical) canon is not the OT bible; there were others, equally Jewish and equally valid: Judaism did not have a single canon. Bingo!
Also, the canon of the DSS contains numerous apocalypses not found in the Palestinian or Septuagint canon. The only thing we know for sure is (a) there was no single Jewish canon; the canon proclaimed at Jamnia and the OT used by the Protestants is not the only Jewish canon (in fact they picked the one that was the least distasteful to the Christ-denying Jamnia rabbis, and therefore the least "Christian"), and (b) the MT p[reserved faithfully the Qumran version of Isaiah, and (c) DSS agree in some parts with the Septuagint, in some parts with the Palestinian (Masoretic) text, and in some it disagrees with both, and finally (d) we do know that the Sadducees' canon consisted only of Torah (but we don't know which version), and that they controlled the Temple.
Good points. It is interesting to note that the Samaritans also had their own "canon", which, not surprisingly, only consisted of the Torah. Recent studies (last 20 years) have found that most Jews actually were reading the Septuagint, even in Palestine, and that the previously held Palestine/Alexandrian canon distinction was pretty much artificial.
Regards