What happened? Market saturation.
What happened to the Photography Industry? The Economy is what happened to the Photography Industry.
... people were moving from film to digital, and that helped sales. Now that the market has reached maturity, it's only natural sales would slow down, and smartphones & the economy just help to roll sales down the hill.
What happened is that the size/weight issue of DSLRs is overrated.
Cameras just can't go more portable than cell phones.
And mirrorless is the wrong solution to the problem of DSLRs being too large and heavy.
These quotes sum it all up.
We are returning to the historic levels of enthusiast and professional camera sales and development. People act like the past decade has been normal, but it hasn't been – far from it.
Normal was the long period from about 1980 to 2000, when the SLR market saw a similar re-alignment after the boom of the 1960s and 70s when every hobbyist bought a Pentax. Canon and Nikon weathered that long period, they consolidated the market and grew to dominance pushing out or marginalizing Pentax, Mamiya, Konica, and all the others.
I would suggest that both companies have the experience and wherewithal to survive and prosper as the market slows. I'm not so sure about the others, although Fuji showed remarkable resilience adapting as the market for their core product evaporated.
If we are honest about it, the current Canon and Nikon full frame offerings could easily last another 5-10 years with only minor improvements. APS-C cameras might need another generation before they are in a similar position, but it is definitely coming. (See some of Thom Hogan's excellent columns on the "last camera.")
Only time will tell if mirrorless is the wave of the future but if today's trends are any indication, it's not looking too likely.