Maybe the smartphone is still causing a dent in camera sales, but I really doubt the people in the market for a sony/canon/nikon mirrorless/dslrwould give that up for a smart phone.
Personally, and with absolutely no evidence to back this up, I think it is more related to the constant deluge of new cameras with marginal features that are pumped out.
There were 7 (numerically) Leica film cameras since the first. There are 6 (numerically) nikon F cameras since the beginning. Now, we see a new digital camera every year or two, which means that these companies have to continue to develop new cameras all the time, just to stay "competitive" and there has been a climax in features. I have a d600 as my digital camera, which does literally everything I need from a digital camera, no matter what nikon throws out there, I have little reason to buy a new Nikon digital.
This is starting in phones as well. The iphone 10(x) sales have slumped, the galaxy s10 sales have been abysmal, and these are top of the line phones. People just aren't going to "upgrade" every single year anymore, the upgrades are not needed.
That, and there is way more competition now.