Can it? Yes. Will it? Only if the NX is a truly massive failure.
Let's say the Xbox One ends up this generation somewhere around 40-50m. The Wii U will be around 14m. That's already 54-64m; making the PS4 need to reach 108-128m to keep its 50% marketshare.
Now, let's say the NX turns out to be a bigger flop than the GameCube and sells only 20m. That would represent a toal of 74-84m; making the PS4 need to reach 148-168m.
You see, even if the NX flops, it will be very hard... unless you think the PS4 will surely outsell the PS2.
Umm, your numbers don't make much sense ...
74-84m/148-168m =/= 50/50
The PS4 would only just have to sell the as much or more than the X1, WII U, and NX combined so why would it have to double them combined ?