Didn't we go through the same thing last gen? Sony simply doesn't have the mainstream appeal they once had with their increasing costs and complex games. They flaunt their horsepower muscle, but as graphics approach being closer to the "good enough" mark, this becomes less and less significant, especially to the average consumer.
Despite Nintendos fall from grace from the Wii days thanks largely to their stubbornness in going back to Gamecube-style niche games, I still think they will maintain a slim lead for most, if not all of the generation, mostly just by default of releasing first. Sure their sales will be down from the previous gen, but I believe all three companies will see decline.
MS will take a close second thanks to their dominance in America and (I hate to say it but) Kinect. Sony will also be close, but will get third. Sony has been in decline while MS has only gained market share, the price of PS4 will most likely be the most expensive, which will hurt them.