You are 1000% correct. The Torah explicitly commands us (Devarim 17:11) to listen to the Rabbanim/Chachamim and not to deviate an iota from what they tell you . Thus every rabbinical edict, for any reason they deemed it necessary is basically a Torah cammandment and must be kept to the tee. Only if the Rabbis themselves said that any particular edict was for a certain time only may we not do it anymore.
It is not up to us as individuals to decide what is worth keeping or not, it is up to the great sages to interpret the law.
The Sages Tell us that Avraham, Yitzchok and Yaakov, our holy patriarchs, kept all the Mitzvos and even all the rabbinical edicts! (See Talmud Yoma 28b)
The Gemara tells us that all the edicts that would eventually be instituted by the Rabbis were shown by to Moshe Rabbeinu on Har Sinai by Hashem Himself!
The Posuk (Devarim 30:12) tells us that the Torah was given to mortals, and isn’t in heaven anymore. It was given to the Rabbis to interpret and to institute fences, edicts etc. as they saw fit.
There is a famous Gemara (Bava Metzia 59b) that describes a dispute in halacha between Rav Eliezer and the Rabbis. Rav Eliezer caused all sorts of miracles to happen to prove his point and that he was right, yet the rabbis were unimpressed, saying that Torah was given to them, and it isn’t up to heaven anymore to determine halacha!
The laws of Rosh Chodesh were given to the rabbis to determine. The calendar month and the days on which holidays fall is not determined by the sun or the moon, rather by edict of the rabbis, as the Torah says (Shmos 12:2) "Hachodesh hazeh lachem- this (and all) months are for you to determine!"
In fact, there are even rabbinic Mitzvos on which we recite "Asher Kidshanu B’Mitzvosav, V’Tzivanu... saying that Hashem commanded us to do it, when seemingly it was only the rabbis! (See Gemara Shabbos 23a and the Ritva’s explanation there)
There are countless more examples of this tenet of Judaism.
Bottom line is that one who transgresses a rabbinical decree has transgressed an Aveira equal to transgressing a biblical decree.
Important Numbers
