The title as written is clickbait.
There's little reason to believe that consumer-grade system cameras (whether SLR or mirrorless) are a dying breed. Sure, most every phone has a camera and many are capable of fantastic output (and I know this firsthand with my Nexus 5X despite a bit of decentering), but there's really no substitute for a large sensor or interchangeable lenses.
What
is dying is the dedicated small-sensor compact camera. The only good reason to get one of these cameras these days is for their zoom capability, and they make a lot of trade-offs in doing so. While I do carry a Nikon 30x zoom compact (COOLPIX S9900) with me most of the time I'm on the field, it comes nowhere near the image quality my K-3 II is capable of delivering especially when coupled with good glass. It is also no match for my Nexus 5X in low light because of the very slow (f/3.7-6.4) lens. The small sensor also suffers from low full-well capacity, with a base ISO of 125, and the camera refuses to use shutter speeds faster than 1/1000s in aperture priority mode unless I stop down the lens at least one full stop (which I hate doing because diffraction kills sharpness on these already tiny sensors), making it difficult to capture good images in very bright conditions. I carry it with me as a practice tool (partly for reasons discussed in
this thread) and to be sure I always have something other than my phone to take pictures with.
Draco