Just because they aren't selling like the PS4 it doesn't means they have to throw the towel. They sold almost 12M consoles and will probably end the gen with anywhere between 65M to 85M units, which is a good number and not that far from what the 360 did. It would be crazy to cancel a product that has these numbers. And going for the logic, Nintendo didn't quit after N64 did 33M and GC did 21M and MS didn't quit after selling 24M OG XB and being screwed by Nvidia.
Not being first equaling a failure is something that only console warriors believe. Having a decent share, 3rd party support, market influence and turning a profit is what matters. If you achieve all of these and not just one or two of them, you are good to continue.
If they somehow decide to quit XB, it won't be caused by the X1 performance. It would happen because the XB division doesn't fit their business strategy anymore. However, with the focus on Xbox Live on Windows 10, X1 streaming and general integration between Xbox, PC and WP, getting rid of the division anytime soon would simply be ridiculous and utter nonsense. Anyone defending these rumours is out of his mind.