The first shot gun I ever owned was a Cooey single in 16 gauge. I could hunt migratory legally with it back then.
That one I bought, but the next one I acquired was inherited from my Dad. It was an 1897 in 16 gauge that my Grandfather bought new in about 1925 or so.
I've purchased many a shotgun since and never shied away from 16 gauge. The first two, I still own. I've added many more:
- a 16 ga, SxS round action, Husqvarna hammer gun (which I have grown extremely fond of)
- a 16 ga Webley and Scott SxS boxlock
- a 16 ga Baikal SxS hammer gun
- a 16 ga Marlin pump (coincidentally called a Model 16)
- a 16 ga Stanford and Laxton Practical Gun and Gun Tool Makers Lefaucheau action just like the "Cat's" earlier in this thread
The last two have since been sold. The first 5 I still own and shoot pretty much every year.
20 years ago, I bought a flat of 2 3/4", 1 oz, 16 gauge. 15 years ago, I bought a flat of Gamebore "Traditional Game" 2 1/2" in 16 ga. I'll be worm meal long before I ever run out of 16 ga shells (also have a small collection of other assorted 16 ga shells bought in individual boxes over the years).
A discussion about the viability of a particular caliber or gauge has to be seen as what it is - a discussion about "commercial" viability. 16 gauge is with us forever; it will never go away completely. There are too many existing guns in 16 ga. and, believe it or not, too much ammo (existing shells and new stock being manufactured all the time). It may even see a resurgence in popularity somewhere down the road (it always was, and still is, more popular in Europe than in the Americas).
If I had to predict the future, I suspect that 16 ga will remain a niche gauge, popular with "aficionados" and purists of the uplands. For the time being, it has been squeezed out of migratory hunting and clay target sports through restricted ammunition supply.
My experience with clay target sports has taught me that there's a place in the world for replaceable trigger groups and freely available loads. But, for me, target sports have never been much more than winter practice. My heart and soul belong to the uplands. It wouldn't matter to me if I was the last man standing with a 16 ga.; I'd make the shells myself.