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.
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 >.<