By default SW collecting will always be niche, simply because of the vast array of options
Boxed or unboxed
Etc etc etc ......
As you immerse yourself in SW collecting you become drawn to certain areas, and collect, then maybe the dream wanes and you sell up and come back later with a quite different perspective
There are so many variables in collecting SW it will never be classed as mainstream.
Personally as well, I'm not bothehered about categorising and putting things in to neat boxes in life, SW is mainstream to collectors in SW and becomes obsessive