I dont think the Gamecubes failure had anything to do with online initially. I owned a PS2 for a long time and never had a single game that was online, i didnt know any PS2 owners who bought an ethernet adaptor for it either. I think the Gamecubes main problem was that it was seen as a kiddy console (the little purple box), where as Sony had targeted a whole new demographic with PS1 which was 20 somethings who were not traditional gamers.
Now the Wii is targetting a whole new demographic again and has relaxed their third party rules to the point where they are a lot like Sonys. Games being released for kids dosent bother me as long as there is enough games suitable for adults too, after all kids are part of the Wii demographic as well.
People make it sound like it is only the Wii that suffers from poor third party efforts but the PS3 is just as bad, it gets shody 360 ports all the time and although they may be big name games they usually are worse than the 360 versions.