I think the XBone has undersold for completely different reasons.
The xbox 360 is a great console and a great media centre. For me, I like the 360 and I play it sometimes buy mainly I use it for TV / Dvd's / casual gaming. A lot of people are like this (which I why MS marketed the XBone in such a way) thus there is no reason to upgrade to XBone one at that price point. This is certainly a bad thing for MS.
However, I think we will see 360 users upgrade at a much slower pace.
Of the people I know who own a ps3 (and have never owned an xbox product) they have been faced with constant problems (like games not being able to save or games completely broken with online updates), i'm not surprised they have upgraded to ps4 at such a rate. Fanboism at its finest.
Lets not loose sight of the fact that MS came out of nowhere with the original Xbox and put sony to shame, stealing a large market share in very little time. All things considered the xbox family tree has been a PITA for sony since day one and I don't think that is going to change.
TBH, I tend to hate on sony but both consoles are a bit pooh if you want to be objective. Neither are really pushing boundaries.