As usual, you miss the point.
First of all, your paraphrase of this narrative leaves much to be desired (possibly because
you appear to have lifted it wholesale from a comment on another website). The point was that, at the end, when Rabbi Yehoshua (arguing on behalf of the Sanhedrin against Rabbi Eliezer) refutes the Bat Kol (Heavenly Voice), he quotes,
lo bashamayim hee ("It is not in Heaven,") referring back to Devarim/Deuteronomy 30:12, the very section you were misinterpreting earlier.
Rabbi Yehoshua's prooftexted point is that the jurisdiction of Heaven to decide individual issues under the law was given up by God when the Torah (both Written and Oral) was handed down at Sinai, because that jurisdiction is part and parcel of Torah. It was God's choice to give us the Torah: we didn't steal it from Him. So since it was a freely given gift to us, God cannot then decide to interfere with its interpretation and application. Rather, He also must abide by the rules He gave us in Torah, which include the rule that we must incline after the majority in legal debate, and that it is the rabbis-- who are the "judges which there shall be in that time" spoken of in Devarim/Deuteronomy 17:19-- who have jurisdiction to interpret Torah and decide issues at law.
You (or the idiot whose bad paraphrase you quoted wholesale) missed posting the final bit of the story in the Talmud, where Rabbi Yehoshua runs into Eliyahu ha-Navi (Elijah the Prophet), and asks him what was going on in Heaven when all this took place in the Sanhedrin; Eliyahu answers him that God laughed, exclaiming "My children have defeated me [at debate]!"
There is a long and unfortunate history of false prophets working wonders or doing sorcerous tricks to support their misrepresentations of divine word or misinterpretations of Torah-- much like Jesus apparently did. Therefore, the Rabbis make clear that working wonders is not a legal prooftext. Even if one can persuade a Heavenly Voice to confirm one's opinion, that is also not a legal prooftext.
Because functional systems of law cannot be based on random claims of prophetic messages. Functional systems of law require ordered systems that can be relied upon to produce effective legal results, whether you have prophets (real or false) around, or wonders or neither. Prophets and wonder workers are notoriously unreliable when it comes to always being around when you need them, and when it comes to everyone being satisfied that they are "the real thing." But law should be law no matter who is around, and it should be something able to be discussed and agreed upon by anyone sufficiently qualified.
If nothing else, this goes to the principle expressed in Vayikra/Leviticus 18:5,
et mishpotai taasu v'et chukotai tishmeru lalechet bahem va-chai bahem: ani Hashem eloheichem ("Follow my laws and my observe my legislations, doing them all, and live by them: I am Hashem your God."). In other words, one must be able to observe the commandments in such a way that everyone can understand them in agreement with one another, so that we, as a society, can live by them. This requires an ordered system of laws. Jurisprudence by random spiritual ecstaticism is not a workable model by which a society can live: to think so would be foolish.