Gn15 is a very niche market. This means trade support is going to be minimal, and mainly aimed at kit/scratchbuilding. Using standard OO/HO mechanisms and chassis does make it a good entry point for anyone wanting to bash a few models. As skills improve people tend to want more detail, and therefore are more particular. There are also a few people who want to skip the basic step. If they have the money then fine, but as I said it is a niche market.
Being a niche market makes 3D printing a better option, but it is not cheap, and therefore better for those modellers who have the money, especially if they ant all the bells and whistles!
One big advantage of the bigger scales is being able to add more detail, and add other features(working). This is what I am working on at the moment, and is possibly a better selling point for modelling small layouts in big scales.
It does seem that we speak more about the trains and less about other parts of the model, although there have been plenty of topics on layouts, which I find more interesting on occasions.
7/8th modelling is also a niche market, and I have noticed some have given up producing kits, presumably because there are not enough people buying them. Another possible for 3D printing, especially as for non railway specific items it is close enough to 1/12th scale to increase potential market. I up sized some of my wagons and you need some serious money to buy what are pretty close to r2r models. Probably why some still prefer to modify G gauge locos and wagons.
Narrow gauge Francophile interested in 1m, 60cm,50cm , 40cm and smaller gauges . Build in scales from 1/6th to 1/24th. Also 1/32nd and 1/35th using 16.5mm track to represent 50cm and 60cm gauges. http://www.rue-d-etropal.com