Simple question. Do you think the Xbox One will have more lifetime sales than the Xbox 360.
Personally I don't think so. I feel the real reason why the 360 got so popular in the first place was due to Sony's many mistakes at the start of last gen with the PS3. Now that Sony has rectified those mistakes with the PS4 and has a pricing as well as a hardware advantage on the Xbone they can retain for the rest of the gen, I feel the PS4 will take the Lion's share of the market.
Assuming the ~160 million who bought the PS3 and 360 are back to buy a new console, the best case scenario I see for MS is them getting 50-60 million and Sony getting 100-110 million. So yeah, basically the Xbone won't be selling as much as the 360's ~80 million, not unless somehow the market expands radically, which remains to be seen but is unlikely imo.
