The Gamecube came after a bad gen for Ninty... They also didn't have the same 3rd party support to help them out, Sony isn't losing 3rd party franchises... And, the core Ninty audience was all that was basically left outside of ultra-hardcore-buy-it-all gamers...
Sony's prior sales is over 120million units sold to users... There's a lot of people out there that bought into Sony's machine repeatedly after 2 gens... There's also a high amount of sales STILL going on with the PS2... Sony needs to get the titles that their fans love out there to get them to move on... There's no FF, no MGS, no GT, no Socom, no rachet, no KZ, no GoW, no KH, no SO, no etc yet... There's a lot more million selling franchises out there that didn't hit yet... And, the only 1 hitting soon is Rachet... Unreal "may" end up pulling some sales... If Socom hits, that'll be big... If the fans get confused over GT5
thinking it's the final product, or it's justified as one, it'll impact...
Sony's sales will pick up once these titles hit, there's not a doubt in my mind... It's just a ways off yet... They also need to have a great TGS for the media to pick up on and show off what's on the way to generate some hype...