The LXX is fine as long as it follows the Hebrew text --- after all it is only a translation, right?? The OT was originally written in Hebrew not Greek, right???.
At a merely human level you are right, but the same that applies to translations applies to scribal copying and redaction of texts. That the Qumran texts are often at variance with the Masorete makes it clear that the issue applies to it as well as to the LXX.
However, you are appealing to a particular theory: first
that an ur-text, directly inspired by God, actually exists, that the Holy Spirit's "[speaking] through the prophets" applies not to the text received by the Church in her councils, but to that now-lost text. And second, merely because it did not involve translation into another language, that the Masorete respresents a more faithful preservation of that original text than the LXX.
Orthodox, Latin, Coptic, Ethiopian, Armenian, and possibly Chaldean Christians (though they favor the Peshita, rather than the LXX) will all dispute both parts of that theory. Only the adherents of Christian confessions whose roots reach back at most to the 16th century, who wish to rewrite and abolish parts of Christian tradition as it was received by all of the confessions that can trace their root back to the first four Christian centuries, and secularized or half-secularized scholars in love with the idea of an ur-text insist on this theory.