That's not why it was regulated to curio status. You can thank the competition world for that. Hunters have always loved the 16, but when it was left out of the competition world, people moved towards the bores that could be shot in competition. That's why there are so many classic 16's on the market compared to today's shotguns.
How is the competition world to blame for that? One can use a 16 ga in lieu of the 12 in Skeet, trap and sporting clays etc. You can use a smaller gauge in any larger gauge event, you just can't use a bigger one. There is noting stopping you. I know/knew a lot of folks who shot their 20s in the 12 ga events. If anything, one can thank skeet for keeping the 28 ga alive. It didn't kill off the 16 cause there is no rule forbidding them from being used in 12 ga events. There's just no advantage to them. One ounce loads from a 12 or 16 at 1200 fps = broken birds. So, why not shoot the 12, which is far more versatile and less expensive to buy?
Almost exclusively competition shooters are using heavier shotguns than most want to carry in the field. So yes, you can use them in competition, but if you are going to shoot 1 ounce then it might as well be a 12 in the 12 events. Even the other sub-gauge competition events are usually shot with 12ga weight shot guns. The vast majority wanting their sub-gauge competition shotgun to match as close as possible their 12 ga in handling dynamics. Either sub-gauge on 12 frames, tubed or with carrier barrels. As always there are exceptions, but the serious shooters try to match them up.
The 16 shines in the field on lighter weight frames and offerings. There are good shells out there for the 16, but you usually have to order them. Which isn't a big deal. When it comes to good shells Winchester, Remington and Federal are not the place to look.
Some have been predicting the death of the 16 since at least the 60's. It is still here....
