The decline started way before that. This isn't about Sony the videogame console company, this is about Sony Everything.
Their consumer electronics divisions have been struggling for a long time simply because they've failed to acknowledge that the market has changed. They completely missed the boat when it comes to digital audio players. Walkman has been replaced by ipod. They took forever to move Bravia into LCD/Plasma/Widescreen, losing almost all of the brand recognition they once held in the TV market.
During all of this the way people shop for products has changed. Fire up a browser, read 10 reviews, place an order. Much different than walking into a department store, seeing a samsung, hitachi, and sony right next to each other and going with the Sony because of brand recognition. The "premium" consumer electronics market is deader than a doornail, you're only as good as your latest model/review.
The divisions of Sony that stuck to "business as usual" during these changes are the ones that have been and will continue to struggle.