Everything about Gen 5 and onwards feels like a reboot that fails to capture the original essence of Pokémon, the most obvious things that jump out are the departure in mon design, music design and from Japan as a setting, but it all feels off and there's a thematic dissonance throughout. Take for example the gangs of each game and how their designs fit the theme of the setting and goals of their leaders. Gen1&2 have petty criminals/gangsters which fit their "goal" of being criminal gangsters, Gen 3 has Pirates & Cave dwelling cultists which fits their respective motivation of worshiping either box legendary, Gen 4 has UFO style spacemen that fits the time/space theme. Gen 5 has... PETA knights that are led by guys who look like midieval JRPG fantasy characters? How does this fit their goals of freeing all Pokémon? How do Knights in chainmail fit the very cosmopolitan American setting? At least Gen 6's fashionista villains fit the France setting, even if it doesn't much fit the box legendaries or their goals.
All this before mentioning some of the battle gimmicks that were introduced after Gen 4, like mega evolution and other mechanic changes later such as the inability to "catch em all." The series feels like an entirely different beast after 4, and it was probably intentional considering B&W straight up didn't let you catch old mons before the endgame (something B2&W2 heavily overcompensated for by having all the older fan favourites shoehorned in easy to get areas).
Plus Gen 4 closes the series off very nicely, with the final dex entries being literal Pokémon gods and if you consider HGSS to be the "last" entries to the series, Red being the final battle, taking you back to your very first adventure in the world assuming you were playing from the very beginning in Gen 1.