Videomaker – Learn video production and editing, camera reviews › Forums › Cameras and Camcorders › DSLR’s › DSLR vs Mirrorless vs Smartphone cameras & diminishing returns › Consider the hammer: claw,
Consider the hammer: claw, ball-peen, sledge, framing, tack, mason’s, driving, etc. They’re all tools and they all could be used to drive a small nail, although many would do the job better than others.
The real key to answering the question you pose is the nail — in other words, what’s the specific job to be done. For traveling light, producing video that may be wobbly and poorly framed, the smartphone is adequate. For most folks going on holiday it’s all they need. I just finished editing three hours of safari footage shot by a client on his smartphone and it looked o.k. The still shots were as good as the photographer.
For more serious still photography the G7 and D750 are easier to hold than a smartphone, have more options for focus and iris control. Both can produce .RAW images as well as .JPG; the G7 is 20 megapixels, the D750 25. An external audio recording device with either will provide good audio. Both will provide excellent video, given the limitations of the cameras.
For professional video, I would choose a dedicated video camera, for all the reasons you suggest. These are not merely add-ons but rather the essence of good video tool. A dedicated video camera provides controls that are readily accessible — i.e., not buried deep in a menu structure but on the surface of the camera — and enables the operator to choose quickly the optimum setup for any shooting situation.
Along with two Sony NX5U video cameras which we use for video production I own a Nikon P900, which has a 28mm to 2000mm lens. It’s great for architectural still photography when mounted on a tripod, but it takes lousy selfies! For those I use my cell phone.