I know Sega been pretty dumb in the past, but come on Gamecube audience and Sonic is a good match. I just hope they don't ruin that because of Sonic Heroes. And they really should make another Sonic games using non renderware engine exclusive to GC before this generation is over.
but they aren't loosing any gamecube sales by porting a game that's on the gamecube to other consoles, they are just gaining sales from those other consoles. the cost of porting would be less than (lackluster they may be compared to the GC versions) sales they could reap. why do you think rockstar ported gta3 and vice city to xbox? do you think the xbox sales of gta3 were anywhere near what they were on ps2? of course not, but they got extra money for doing an insignifigant amount of work.
renderware in itself was just an example of how cross platform everything has become this generation. middleware is huge. sega could very easily license another middleware solution or create their own (they have experience on all 3 magor platforms and pc).