Well when the tiny area of interest, phone cameras, is magnitudes more prominent than the wide area of interest, cameras I presume, what is that telling you about what is niche and what is mainstream?
You are confusing two things here: number of cellphones around and their use as snapshot machines; the number of cameras around and their use as serious tools for making photographs.
Those are two very different things. In that wider perspective, the use of cellphones as tools of choice for making said serious images is tiny: the niche which is giving you concern.
There was ever the snapshot "photographer" who never was a photographer in the first instance, just a kind old person looking for "I was here," imagery.
The serious amateur photographer was never of that ilk: he was sometimes a very talented practitioner of the art and, often, better able to afford expensive equipment that was his professional counterpart. (Not all pro photographers get rich.) Others were just interested in the process, and wanted to make better graphic memories than they were getting from the chemist. That group, collectively, is what the market for cameras is made from today as yesterday.
Unavoidably, the guy who has but a cellphone, is hardly thought of as a photographer, so he cannot be part of the massive group that you would dearly like to use as basis for your assertion. That being so, there is no tiny niche for cameras: they are the name of the game. Attempting to subdivide them further is silly, unless you are a manufacturer wondering what to do next.