I haven't really thought this far ahead, but it seems possible that the PS4 could reach around ~50 millions by the end of 2016, X1 around ~30 millions and Wii U around ~20 millions.
I made a quick and simple comparison between the PS3 and the PS4. The numbers below are roughly however, not exact and taken from VGC.
PS3 sold 1,2m during launch Nov-Dec 2006.
PS4 sold 4,4m during launch Nov-Dec 2013.
PS4 lead of 3,2m.
PS3 sold 1,9m between Jan-April 2007.
PS4 sold 2,9m between Jan-April 2014.
PS4 lead of 1m.
PS4 total lead of 4,2m to the PS3.
If this continues, and the PS4 would outsell the PS3 and keep around the same 71%-lead, it would mean that the PS4 would have sold 54,7 millions by the end of 2016.
The PS3 had sold 32 millions by the end of 2009.
I think my calculations are correct. :P
So my bottom-line is, if things keep going as good as it is for the PS4, and the above would apply, then yes it could make 50 millions by the end of 2016.
But the market can change a lot, economic crisis, loss of interest for the PS4, growth of Wii U or X1, and no one can really know what will happen so far into the future.
My own estimate would be somewhere between 40-50 millions. So for me 51 millions are a bit bold but perhaps possible.








