Well, it did occur to me that this long-standing debate is like fighting over whether Hamm's or Pabst is the better beer...
The reason the Model 12 is not made any more is because it's a PUMP GUN!
It was popular for American Trap, but serious American Trap shooters quit using pump guns in any appreciable number many years back.
Bird hunters with discriminating tastes still love a particular pump gun, and it's neither one of those.
For everyone else, a pump gun is cheap, reliable and versatile. Throw out any of those, and there's no reason to buy the thing.
WRT an 870 being junk, well, objectively it
is junk: stampings and piano wire. The thing is, it's one cleverly-designed pile of junk, that's good for hundreds of thousands of rounds.
Anyone can make a good gun for a lot of money. Remington figured out how to do it for not a lot of money. Mossberg followed suit. And the Model 12 found few buyers after that. Sad, maybe, but true.