I am looking for a serious answer as to why fasting on the 3 minor communal fasts is treated as undebatable today. When the Shulchan Aruch states that no one should digress from the universally held tradition - it can be said that the majority of Jews today do not fast. Why only count those who observe the Shulchan Aruch, as opposed to opinions (Rav Papa in RH18:) who would say it is optional? Is the minhag of the klal no longer relevant? The klal certainly behaves as if the minhag is no longer relevant today.... Thank you

The custom here, as always, is set by religiously observant Jews, not by those, albeit the majority, who on the whole, unfortunately know next to nothing, and usually don't care whatsoever about Judaism and the Shulchan Aruch. All will agree that it would be self-defeating if Judaism were to say: "the custom is for Jews to act like gentiles and ignore Judaism"! Especially regarding these fast days where the very goal is to improve and do Teshuva. Yes, originally if the Jews wouldn't have accepted them, they wouldn't be obligatory, but once they were accepted, they are like any other mitzva. Most rishonim agree with the Ramban and Tur et al, that as long as we have no Beit HaMikdash, it's not called a "time of peace", and these fasts are still obligatory. This is the decision of the Shulchan Aruch as well, which obligates us all.