There's an old expression, "If it ain't broke, don't fix it." Nintendo tried to fix Super Mario 3 when they were making Super Mario World, and in the process ruined it.
Um, fix?
Super Mario World is a completely different experience from Mario Bros. 3. I doubt they were trying to emulate it or surpass it. It was its own adventure with its own set of challenges.
The problem I see here is that Mario Bros.3 was too good of a game, and thus people view the games that came after it as inferior efforts.
Mario Bros. 3 for many is the highlight of their gaming years. It redefined Nintendo and turned Mario into a pop culture icon. The game was the best moment in the movie "The Wizard" and its secrets were legendary.
It was so, so good that it would be hard to top it, hence why people see World as being inferior, even though its a high quality title with lots of replay value, creativity and challenge.
Not saying that Mario Bros. 3 is overrated, but it did create an act that would be hard to follow, and to this day its still hard to beat.
So if you were to compare every Mario game to Mario Bros. 3 of course all of them are weak. But if you were to look at each title as its own adventure then you would learn that all of them are fantastic.