The Xbox One X has a niche demographic, it not only caters to mostly just Xbox fans but also caters to those willing to splurge their money on 500$ hardware.
The Xbox One X was never made to be a sales juggernaut like it's older counterpart, just a mid-gen refresh that is a prototype for the next Xbox, seeing how it does on a 4K screen, whether people are willing to buy the console just for their word of 4K and "more power". This console also acts as a catalyst for bringing the 4K screen to the top of people's to buy list so that they are ready for the next Xbox.
The console was an experiment and by the looks of it, the Xbox One X did succeed for the most part considering its demographic, I mean it did have a strong holiday presence thanks to its incessant marketing it was more successful than Microsoft imagined.
So yeah, it's a success but it remains to be seen whether the console will have legs beyond the holiday season. It's imperative that they do deals with games like Anthem and the new Battlefield, bundle up those games with the X to boost their sales.