I've killed more dove, ducks, and geese with the 12 gauge than anything else. It'll work fine for anything out there. I've killed a lot of ducks with the 20 back when lead was legal, 2 3/4" number fives being my favorite load. But, there is no denying the 12 gauge is a much better waterfowl gun, especially with the advent of steel shot. And, there's still a spot for the superior payload and pattern of big steel from the 10 gauge in goose hunting.
I sold my 20 gauge when I was in college after I bought a nice 6.5 lbs 12 gauge SxS. Yeah, with 3" BBs on geese, that gun kicked, but I handled it. It took a lot of doves, too. I bought another 20 several years ago, an even lighter, quicker SxS. But, I can still do damage on the dove population with my 12 gauge Winchester autoloader. It's a gas gun and with 1 ounce dove loads, it kicks about like a 7/8 ounce 20 in a lighter gun. It's fairly quick to the shoulder, too, but doen't change direction as quick as the little 20. All that is nuance way I look at it. I can get it done with the 12. If I hunted much quail, I'd do it with the 20, I think, but I don't. I'm a waterfowler/dove hunter.
12 gauge just plain works, that's why I have 3 of 'em. But, I've rediscovered the pleasures of the 20 gauge on doves these last few years. Ain't takin' it goose hunting, though.
I have shot teal with it, 3" steel 4s. It works, but 2 3/4" 12s work better.
My old 16 is retired. There's no steel shot for it and hard to get ANYthing for it, really. Mine's got a 30" full choke barrel, was a great goose gun back in the day, but not much worth anything else. Why should I buy a 16 when the 12 is better? I liked the 16, but really, I have it covered with 12s and a 20 way I look at it. I don't dislike the 16, just don't see the need and the ammo obstacles are a little much.
I don't think the .410 and 28 are "doomed". Heck, the 28 has always been an oddball. The .410 was my first, took my first dove with it. I think it makes a good kid's first regardless of the thin patterns. By golly, if you learn to hit with it, a 20 gauge us budda. ROFL! No, the 10 hasn't died, the 28 has always lived in obscurity, the .410 won't die, either. The 12 and the 20 will continue to rule the day. 16? Well, I don't know about the 16. I ain't counting it out, though. It's STILL more popular than the 28. 32 gauge, that one's gone.