Why are we bound to rabbinic laws?

Question

I know that one of the reasons given for our being limited by miderobonon laws is explained in Masechet Shevuot, 39a which says as follows (lifted from dafyomi.co.il)

i. "Asher Yeshno Po ha'Yom" refers to those who stood on Har Sinai. "Asher Einenu Po Imanu ha'Yom" refers to future generations and those who will convert later;

ii. This applies to Mitzvos received on Har Sinai (i.e. mid'Oraisa). We learn Mitzvos (mid'Rabanan) that will be given later, such as reading Megilas Esther, from "Kiymu v'Kibelu" - they fulfilled what they already accepted (i.e. whatever Mitzvos Chachamim will enact). (end of Beraisa)

But the braita is brought down with the question: "How do we know that we are obligated to commandments that are in the future, to be innovated, SUCH AS MEGILLAH?" (emphasis mine). The answer is given from the megillah, itself.

Ignoring the potential self-supporting loop (the megillah is authoritative because it says it is) by invoking the idea that Tosafot refer to, that the text is divinely inspired, this leads me to 2 questions

1. Isn't the answer provided by the braita simply a local rule? The question is about megilla and the braita explains why we would be bound by the law of reading megilla! From where do we generalize this to anything else? If the question was about megilla and I had a proof from a different source which I could generalize then I could see the larger application, but when the answer points only to the example in the question why should I apply it to other cases?

2. How does the answer apply to any edicts which came about AFTER the time of Esther? If the Jews (some? were all Jews living in Shushan and its provinces then, in the same way that ALL Jews were at Sinai -- the point the Braita started with) rose up and ratified their Sinaitic behavior during the megilla then they were accepting the things that came up between matan torah and that moment. Until we all "rise and accept" today, why are we bound to anything innovated between then and now?

Answer 1

Deuteronomy 17:11 על-פי התורה אשר יורוך, ועל-המשפט אשר-יאמרו לך--תעשה: לא תסור, מן-הדבר אשר-יגידו לך--ימין ושמאל

Deut. 17:9 & 11, (Chabad Link): (9.) And you shall come to the Levitic kohanim and to the judge who will be in those days ... (11.) According to the law they instruct you and according to the judgment they say to you, you shall do; you shall not divert from the word they tell you, either right or left.

This verse is understood to grant legislative power to the Sanhedrin for all time.

There were Sanhedrins in session up to several centuries after the time of Esther and that is where all fully-binding rabbinic Law as we know it comes from.