Nev,
“Clearly nobody in the real world holds that holding by Rabbeinu Tam’s tzeis or 16 amos reshus harabim is gaavah”
As a BTW, i don’t think the two issues you mentioned are comparable, as R”T is really the shitah of the Shu”A, Magen Avrohom and many others, it is rather the other way around that many had the “minhag” not like R”T, whereas, regarding 16 Amos, all Ashkenozim relied on Rashi and is the Halacha of the Shu”A, Rema, MA, T”Z, Pri Megadim etc.