First off, do you think that will be so world wide if you only count the PlayStation and Xbox consoles. The Wii and Wii U can be safely be excluded because they don't pull the same types of games. By that I mean that the Wii U's tanking won't have any effect on Activision's decision to make Call of Duty or Destiny. So yes you can say that the overall market will shrink, but it's effects won't be evenly distributed.
Second, we have seen how price sensitive these consoles can be. This generation is the first where every hardware maker did not subsidize their console. That is going to have an impact on sale. If the consoles had come out $100 cheaper or had $100 worth of extra hardware at the same price, sales would have been much higher. What this means is that we'll have a delayed uptake as the prices naturally drop.
Third, continuing with the price theme, this console generation has existed entirely under a weakened economy after one of the biggest recessions in history. As/if that corrects itself console sales will rise.
Fourth, as I pointed out before there is a HUGE discrepancy in the number of console selling games out right now from the PS2 era. This could be a systemic problem due to the higher cost of developing games, so this one might not correct itself. However it could also be fluke. We could have been held back by the trend to make cross gen games, and remasters. Now that that is at an end we could start seeing more games released worthy of buying a new console to play.
Finally, both the XB1 and the PS4 will get a sales boost when they release Project Morpheus and HoloLens respectively. Either or both have the potential to hit fad status and sell like the Wii U's motion control or Kinect.
There are just way too many variable that we know will change to project a few month's worth of data out to the lifetime of this console generation.
Historically, all these attempts to suggest game changers are coming have fallen flat. It is nearly historically unprecedented for a system to dramatically change its course after 1.5-2 years on the market. At this point last generation, Wii was on pace to be the clear number 1, and PS3/360 were fighting a very close race (within 8 million of each other) for second place -- and that's exactly how the generation ended. People suggested that the Gamecube could turn things around, or that the Xbox could, but that never came close to materializing either.
It doesn't mean it can't happen this time, but most of the time when people think game changers are just on the horizon -- price drops, peripherals, what have you -- the best they've done in sustain sales in the long term, not alter the trajectory of the generation in significant ways. Possible, but I'm not betting on it.