I'm not entirely sure it's such a good thing to go after the Sonic fanbase and blame them for the fall of the blue hedgehog. After all if it wasn't for them the series would have just died after Sonic '06. Those people are the hardcore of the hardcore, a cultlike entity that will happily absorb anything and everything Sonic no matter how bad it might be or how much it was rushed, and for better or worse they embraced Sonic and allowed it to survive to this day. Not all of them are like that of course, but still, it's worth noting.
It's more Sega's fault than the fanbase, but both are to blame in different ways. Sega has a habit of rushing things, and this screwed up Sonic more than anything else. Wanna know why Sonic '06 was so epically bad? It was rushed. Why was Sonic the Hedgehog Genesis such a failure? Rushed. Sonic 2 was rushed, yes, but it was still well built and a wonderful game, and it didn't rely on any stories or myth arcs or anything like that. Just good gameplay, which allowed it to get away with several planned levels being cut so it could be released in time for Christmas.
But of course being rushed isn't the only reason why Sonic began to fail. After it left the Genesis Sega began to lean towards more storytelling in Sonic games, and this doesn't always work well for two reasons; the first being that the cast is made up of silly looking furry animals and one tall egg-shaped mad scientist, and the second being that the stories are rarely if ever thought out well. It takes time to make a good story, because it takes quite a while to work out all the kinks and plot holes and make it nice and sensical, and this is rarely if ever done for Sonic. Which is fine; the Genesis games didn't have a storyline beyond "Dr. Robotnik is doing evil crap and Sonic has to go stop him along with Tails/Knuckles" but it really didn't need a good deep storyline to be good. But they attempted to go deeper and ended up with progressively more and more poorly thought out stories that never got the opportunity to be refined, and this dragged Sonic down like a cement weight drags down a victim of the mob in a raging river. Sonic wasn't ever really meant to engage in Final Fantasy or Zelda-esque storylines, but they tried it anyway, and it just never really worked out, both because the stories weren't thought out well and because they didn't really fit the characters involved. But it's not like Sega took notice of this; they kept trying it and trying it and it's only recently they decided to scrap it and just go back to what Sonic was originally about; going blindingly fast through levels and smashing up robots.
But it's partially the fans' fault too, because they bought this poorly thought out and badly beta-tested crap and actually liked it. I have nothing against you if you like Shadow, or if you like Sonic Adventures or anything like that. Everyone has their own personal tastes. But you can't like Sonic '06 or Sonic the Hedgehog Genesis (barring the glitches or laughing at how ridiculous it is of course) and have good taste in video games. They were rushed, horrible mockeries of video games, only printed so Sega could profit from them. But those hardcore Sonic fans not only bought them, but still defend them to this day, along with other bad Sonic games, and Sega pandered to them for quite a while before deciding to just screw it and go back to the basics.
That girl is really hilarious though.