In the last 7 years, I've seen 104 people in my office (out of the 148 total staff) who bought a DSLR ... but only 9 of them are actually doing something with their DSLRs. I have an American colleague who bought my entire kit (DSLR, lenses, tripod, flash etc for $2500) in 2009, but he hardly ever uses any of that gear (I think the last he used was over 2 years ago). I think that does reflect the vast majority of DSLR customers in the past 5 years or so ... a lot of them bought DSLRs thinking that it will somehow magically transform their images, because they see other photographers take great images with DSLRs. But they do not understand the basics of photography (i.e. f/stops, aperture, ISO etc) or have the interest in post processing ... most of them use the auto mode and get frustrated that the flash keeps popping up at the most inappropriate times or their photos, from their very expensive DSLR/lens, don't look any better than their smartphone and not many are willing to carry the weight/bulk that comes with DSLRs ... eventually they give up, some sell it off, other let it collect dust ... the DSLR sales "peak" we are referring to accounts for a vast majority of people who have bought them and pretty much never used them after the first few weeks or months ... basically it was the beginners who were caught up in the hype. Lets face it, DSLR photography is an expensive hobby and the upgrades are never ending ... not everyone is willing to keep on upgrading like some of GAS sufferers.
So, now, it the more serious hobbyists / professionals who are buying/upgrading DSLRs and not the vast majority of beginners (who used to buy till a couple of years ago).
Just the day before yesterday, I gave a ride to an English lady during a bird watching trip (organized by a famous birder), and she bought her Nikon D300 + 18-200 lens 5 years ago ... she told me that she shot less than 1000 photos with it, she says that most of her images don't look any better then her iPhone 4S. So she just uses her D300+18-200 once in a great while, when she thinks she might need some reach. So why should people like her, be upgrading to an expensive DSLR/lens ... so they basically stopped buying/upgrading them ... I think that is the biggest factor in the loss of sales, the economic slow down etc are just secondary factors.