The problem with this is exactly as you said; they've failed at properly reintroducing these characters so people actually give a damn about them. Tails, Amy, Knuckles, Eggman, and even Shadow all can get by because they're just that ingrained in the public consciousness of the series. When you think of stable Sonic characters, Sonic and those five will generally come to mind. If you had a Sonic game that starred Rouge and the Chaotix, the general reactions would probably fall along the lines "Who the fuck are these guys, and where are Tails/Knuckles/Amy/Shadow/Eggman at?" This is what happens when you have too many characters, and fail to make people actually give a shit about their existence. It's telling that Shadow is the only post-Classic era character that people generally remember, when Rouge debuted in the same game as him.