I think once the smartphone's internal capabilities are exceeded, a "System" (as in put together from multiple parts) DSLR or Mirrorless has more advantages:
- if you need more parts, eg. flash, you go from having one thing in your pocket to carrying a bag anyway
- dedicated UI and buttons for adjustments while shooting without having to look away from your subject or having to change your grip
- much larger sensors means more light to work with during shooting and post-processing
- dedicated glass that you can't quite yet replicate with light-field tech
The smartphone can do many of the things a dedicated camera can, it's just not as good on almost all fronts, and much worse in some aspects. You can under more and more conditions get images that rival DSLRs, but not ALL condititions: If you can control time, light, and subject all at once, a dedicated camera can be matched. If you can't control of only one, grab a dedicated camera.
The tactile UI is one major gripe against smartphone photography for pros and enthusiasts alike, which is why a phone to some extent needs to be smart/automatic, and while today's image sensors straight beat the pants off any predecessing image sensors, physics still poses hard limits on noise and light capture.
Today's image sensors are very close to be able to count individual photons, and making the sensor larger means being able to capture more of them at a time. Tricks are being worked on to extend dynamic range and lower noise (like double exposure HDR), so image quality still increases, but the larger image sensors profit from those developments just as much as the small phone ones. The days of small image sensors being good enough to beat a human eye are still far off.
TL;DR: dedicated tactile UI, physical interfaces, physics, can't quite be beat by all the high-tech we can pack in a smartphone package.
