sales of lots of things went down in 2015, reflection of the economy in general and also that there really was not any major advances in camera tech introduced in 2015. We did not see a huge jump in MP count, High ISO quality or any other tech superiority that we get every few years. Like the move to 30-50 MP sensors or a couple of years back.

Eg the Nikon D800 is still 'current tech'. Lots of people will not upgrade until something comes along that shines above the current crop.

So a slump in camera sales is just a reflection of the current product and other non-camera related factors. Fairly much less of everything sold in 2015. Mobile phones were probably the exception. But even then iPhone sales dropped and continue to do so, as an indicator as well.