It really depends on how long this console generation last. Right now though, X1 is on track to match the X360's sales from last generation.
Actually right now it is about !M ahead where the 360 was at same time in its first year.
People try and compair the X1 to the PS4 to show why its "doing so bad". But the reality is that if you look at the Xbox One in a bubble, it is doing very healthy sales wise. The XB 360 sold about 5.9M in its first full year (November-November), and curently Xbox One is on track to do the same if not better.
Now if you are tring to say that the Xbox One will never match/catch the PS4 in sales globably, then I think thats an easy prediction as not even the 360 was able to do that to the PS3 globally. If you are just talking about NA though, I think you are underestimationg how strong the Xbox brand is in NA.
At price parity it will most likely sell about the same as PS4, but if they undercut the PS4 (I think they will) then I think they will outsell them in NA.
