Very telling graph. While DSLR sales are still higher than mirrorless currently, DSLR's have been in a state of decline for three + years, and mirrorless sales have been growing for the past two. If these trends continue, mirrorless cameras really will start to replace DSLR's. Canon and Nikon will have to position themselves in market segments that are actually growing eventually, and I would be suprised if we don't see something from either of them soon.
Rough math...but based on current sales and the growth/decline rates of the two segments....mirrorless camera sales will surpass DSLR sales by 2019 (assuming growth and decline rates are stable)
The problem for CaNikon now, however, is that Sony has solidified their position as the leader in the mirrorless space in terms of technology, brand, and customer base. They shot themselves in the foot by disregarding mirrorless for so long. Even if the two of them do go mirrorless, we will not see the same duopoly between Canon and Nikon that we saw in the DSLR realm. This is a good thing in terms of innovation and pushing the technology and industry forward, as the duopoly between Canon and Nikon, IMO, create a game theory type situation where neither of the two had any incentive to innovate. The DSLR market actually incentived the two of them NOT to innovate...hence the stagnation of the two brands in recent years.