At one point in time practically everyone owned Walkmans, and now everyone owns iPods ... While the Walkman brand may still be valueable and people may not have any problems with what Sony is offering, when most people go looking for a brand new music player they will look at the iPods first (and typically buy one before looking at what other people offer). In order for Sony to return the Walkman brand back to the dominance it once had first Apple has to screw up pretty massively in order to provide the opportunity, and Sony has to produce a product so vastly superior to the other competition that practically everyone chooses a Walkman over whatever else is available.
As you can probably imagine, the combination of needing Apple to screw up and the need for a dramatically superior product is something that only really happens when their is a paradigm shift in the industry; and these are not the kinds of things that are amazingly common.
Basically, Sony is in the same position with the Playstation and Wii as they're with the Walkman and iPod ... While Sony can chip away at the success of the Wii it is unlikely that they would return to the same dominant position with the PS4 that they had with the PS2 unless Nintendo screws up and they can produce a product that is dramatically superior to the XBox to the extent that practically everyone favours a PS4 over the next XBox.
