First of all, the topic title is a bit misleading - "No market for 12 gauge?" vs. what you actually discuss - "no market for selling high end 12 gauge double guns for hunting".
There is an ENORMOUS market for 12 ga guns; they are easily the best selling shotgun, and the best selling ammo as well. There is also a big market for 12 ga guns - in the TARGET venue. Krieghoff, Kolar and others don't seem to have a problem making a profit on their definitely high-end 12 ga shotguns.
High end hunting guns are another story. IMHO - "High End" generally equals "well heeled" and "experienced". This market is, again IMHO, leaning towards smaller gauges. They've had enough of the recoil that 12 ga guns and ammo generate, and enough of toting relatively heavy guns.
I do know guys who have hunted for decades and have plenty of scratch, but they still tote a 12 ga because it will kill as many birds (or more) than any sub-gauge. They also generally tote Rem 870s, and I would not consider them "high end" users. They are "basic" (not new) users, who are only interested in putting meat on the table.
Many experienced hunters, though, are no longer strictly interested in "killing" per se, but are seeking the pleasure and experience of the hunt, which they find in lighter guns that are more easily carried and will still complete the task as needed. In fact, if you need to pass up a few long shots, so much the better! That just prolongs the hunt.
I for one rarely if ever hunt with a 12 ga. I do have two in my safe, a Beretta 3901 which I use for occasional target shooting, and a Mossberg 835 that my son prefers for our occasional hunts together. 16, 20 and 28 ga. guns fill my needs very well.
