Realistically, I don’t think they can be unless Nintendo becomes arrogant and screws up. If you look at the modern videogame industry (NES and newer) there have been 5 console generations and 3 handheld generations and the market leading console manufacturer has only switched place twice. In both cases that the market leading console manufacturer was replaced several very large mistakes were made by the previous console manufacturer that opened up the opportunity for them to be replaced; in the case of the N64 it was Nintendo alienating third party publishers and giving the Sega Saturn and Sony Playstation an 18 month head start to establish themselves in the market, and in the case of the PS3 it was Sony alienating the broader gamer market and releasing the PS3 at the ridiculous price of $600.
If Nintendo releases a console in the $200 to $300 range (possibly up to $400), 5 or 6 years after the Wii launched, with decent third party support it would be very difficult for Sony to pass them; if Sony responds to a system like that by releasing a console 12 to 18 months after Nintendo and/or releasing their new console at a price over $400 I would expect the PS4 to have lower sales than the PS3.
