Actually, what you wrote above is not correct.
Orthodox Jews hold that any halakha that came from Hashem is unalterable IF Hashem stated that it is - IF Hashem gave permission that something could be altered then it can be.
Further, Orthodox Jews also hold that halakha is determined by a Mosaic Court in Jerusalem.
In the absence of a Mosaic Court all Torath Mosheh and Orthodox Jews more than 1,000 years ago decided to use the rulings found in the Bavli Talmud UNTIL there is again a Mosaic Court. Lastly, Torath Mosheh and Orthodox Jews hold that when there is again a Mosaic Court in Jerusalem we go by their rulings on Halakha.
One source for what I wrote above is the Rambam's Mishnah Torah - Hilchoth Mamrim chapters 1-3. Here is a part of it:
"The Supreme Sanhedrin in Jerusalem are the essence of the Oral Law. They are the pillars of instruction from whom statutes and judgments issue forth for the entire Jewish people. Concerning them, the Torah promises Deuteronomy 17:11: "You shall do according to the laws which they shall instruct you...." This is a positive commandment.
Whoever believes in Moses and in his Torah is obligated to make all of his religious acts dependent on this court and to rely on them.
Any person who does not carry out their directives transgresses a negative commandment, as Ibid. continues: "Do not deviate from any of the statements they relate to you, neither right nor left."
We are obligated to heed their words whether they:
a) learned them from the Oral Tradition, i.e., the Oral Law, (From Hashem)
b) derived them on the basis of their own knowledge through one of the attributes of Torah exegesis and it appeared to them that this is the correct interpretation of the matter,
c) instituted the matter as a safeguard for the Torah, as was necessary at a specific time. These are the decrees, edicts, and customs instituted by the Sages.
It is a positive commandment to heed the court with regard to each of these three matters. A person who transgresses any of these types of directives transgresses a negative commandment. This is derived from the continuation of the above verse in the following manner: "According to the laws which they shall instruct you" - this refers to the edicts, decrees, and customs which they instruct people at large to observe to strengthen the faith and perfect the world. "According to the judgment which they relate" - this refers to the matters which they derive through logical analysis employing one of the methods of Torah exegesis. "From all things that they will tell you" - This refers to the tradition which they received one person from another."