It's a problem with the XBONE, and potentially a problem with the PS4. Check this image:
Except for the first week, the Wii U was selling better world wide than the XBONE. The last week of Dec. however was when sales plumeted. It was a mix of the hype dying and the end of the holidays. This drop off happens to every non supply constrained console. It'll happen to XBONE and it'll probably happen to PS4.
With XBONE it's in a very similar situation to the Wii U. Lots of stock and low demand but still selling pretty good. The problem is the XBONE is doing best in America. What caused the 360 to dominate during the holidays most years will also cause the XBONE to have a huge drop. Americans love buying consoles during Christmas even more than Europe and Japan. Last year in America the 360 dropped from 470k to 197k during this time. That's down 58% week on week. In Europe it went from 194k to 150k, or only a drop of 23%. Comparing this to the Wii U's sales during this time last year, it dropped from 113k to 60k in America and from 70k to 48k in Europe. 47% and 31% respectively. Slightly different percents, but still showing the same trend of having a much larger drop in America.
Luckily for us, this year is only one day off when comparing the weeks. Week ending Dec. 29th, 2012 matches almost perfectly with the week ending Dec. 28th, 2013. Last week XBONE did 236k in America and 79k in Europe. Using the same drop off ratio's as Wii U instead of 360 (since it still has some launch hype) gives us a prediction of 125k USA and 55k Europe. XBONE did 357k WW the week before so factoring in the other markets with the same ratio we get 204k WW for the week ending Dec. 28th. I'll make the prediction 210k WW to factor in the weeks being off by a day. The drops continue into the next few weeks of Jan. The hype is all but gone by then.
With PS4 it has supply issues which make such an analysis impossible to perform. We can say the demand should drop off by a very large percent, but at the moment we really don't know what the demand is. In addition although only slightly, being a hard to find item can increase demand. US be US though so I just feel like US supply problems will be over in Jan. Hype in US should end around then. Globally though it could be a very long time.