It's not that. Of course the XBox One will continue to sell after the NX comes out. It is the fact that the Wii U will essentially be exempt from the marketshare once the NX comes out(VGChartz did not blend PS3/360 sales into the Wii U/PS4/Xbox One sales) so in essence the PS4 and Xbox One marketshare will automatically rise by default once the NX is factored into the equation and the Wii U is exempt. It will be the NX which will start off at a tiny percentage and have to battle it out.
Let's also not forget to the PS4 continues to outsell the Xbox One by pretty much 100%(double) weekly worldwide so the PS4 is constantly taking up more and more marketshare despite the Xbox One still selling.
True enough. Though if we want to do that... We could argue that the NX has 100% of the sales for gen 9 -_- Just saying xD
But other than that yes, the PS4 feels like an unstoppable force at times >.<