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The only other huge drop in quality was from B2/W2 to X/Y. People forgave misgivings like Megas, short routes, fairy types, shoehorned story, neutered difficulty, Kanto pandering, worst competitive balancing - bar Kanto - to date, terrible soundtrack, ugly and washed out 3D models, and a forgettably bland region because it was the series' first foray into 3D, and things would OBVIOUSLY get better from then on.
But then the games got incrementally worse slowly enough that people didn't really notice the drop in quality, until we dropped from the 4.5/10 USUM to the 1/10 Sw/Sh.