Sonic Heroes isn't necessarily the end of the Adventure series, it's the start of something completely new. They interviewed one of the members of the American Sonic Team in Electronic Gaming Monthly and he said that it's all up to the fans on what they'll do next.
It is kinda funny that more info has been released on Sonic than Billy, but SEGA's banking on Heroes big time, all this stuff, McDonald's, the GBA games, Sonic DX, the Cartoon, it's all there to hype up the release of Sonic Heroes. And also, SEGA doesn't have a lot of trouble selling on the GC(they own 10% of the market share), but I'm sure we'll start to see more hype surrounding Billy Hatcher in August/September.