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Gen 5 was and still is without a doubt the biggest shift in Pokemon in general. After HGSS, the popularity of the DS was waning out and people moved onto iPod Touchs and Blackberry phones. The older fans got their Tajiri remakes and The 2000's fans along with the older 90's fans thought that the series was just about wrapped up. No more japanese esque regions and the power ceiling was already reached with Arceus and post-Pokemania pokemon had pretty much come and gone. When Gen 5 came out people were pissed with how all of the older pokemon were just forgotten in exchange for 150+ mons nobody asked for not to mention the amount of railroading made it feel completely different. It didn't feel like Pokemon. There were almost no spinoffs, no home console interaction, the anime sucked more balls than it ever had and everyone just stopped caring and everyone who did stick around when Black and White came out became disenfranchised with the series and by the time BW2 came out the fanbase was a completely different beast entirely.
The financial crisis had killed 2000's culture completely and Hot Topic with it too. Many people who grew up playing Gen 5 wouldn't know what Pokemon used to be like in gens 1-2 or even 3-4 because the culture had shifted so much.
Also their fanbase is by far the absolute worst