Oh here goes old man Barry with his age discrimination.
I would say it has more to do with people around them, and the communities online. A lot of people into the franchise now started out on the GameCube, but that does not mean their opinions should be considered any less than ours or that they cannot like the older titles as much.
It is like, growing up I was not really a fan of Golden Axe. I mean, I did not hate it, but I just did not really appreciate it as much as I should have. When I got older, I started to learn more about it, how to play it better and in the end it has turned out to be one of my favorite games by the publisher.
Communities can get together and discuss Sonic's marketing, talking about how speed was the main focus and whatnot. This would rub off onto others who read it trying to learn about the franchise with them thinking that is how the games are like, and then in the end it all spreads down to people thinking the series is supposed to be something it is not. Adding to this with how many styles the Sonic series has had, it is always going to be rare for there to just be a fanbase that likes it all.
I think a good way to sum it up is that for me, Shenmue is one of my favorite games. People who have played it may not like it, and people who have just seen it may not like it either, this has come down to some people saying it sucks and communities listening and taking note, thinking that is just the truth. It is the same for me with games like Jet Set Radio and Valkyria Chronicles. Because those fanbases like them so much and will not shut up about them, and keep asking SEGA to just make those I end up liking them less. Does that make me shallow? Maybe!
Also, I hate it when people call it Sonic Adventure 2 Battle instead of Sonic Adventure 2, those dirty bastards. > : [
