That is incredibly bad logic. New movies come out for the same reason new Spiderman movies came out. New directors always thought they could "fix" the problems of the previous movies. Superman movies have been utter failures for decades, this is incredibly apparent just by looking at the highest grossing Superman movie being Batman v Superman, a fucking Batman movie. This is why the James Gunn movie has done so well financially, it was directed by someone that knew how to make a superhero movie and it STILL didn't get good ratings! Yet look over at Spiderman, which had numerous terrible versions until decent directors got a hold of it. Why hasn't that happened for Superman? (hint: it's not because the directors are bad)

And I gotta say, something existing for decades while still being bad has nothing to do with the level of boring. People hold onto nostalgia, they also will keep consuming stuff even when it's terrible. McDonald's for example.