To be fair, the vast majority of Duck Hunt's prominence arises from the fact that it was not only shipped with the system, but on the same cart as SMB itself. That said, you're very right, Duck Hunt was not a "core" game, but that's my point. I've absolutely no problem with the more casual, gimmicky games (like Duck Hunt) to entice people who otherwise might find less interest in the system, I would just like to see those experiences presented as additional to the main experience, not tied to the system in a compulsory manner (as of late through the default controls of the system - with games that use a traditional control scheme being the exception, not the rule). It's not like the Light Gun was the primary controller for the NES itself.
Yes, in the west, due to the reticence of the gaming market after the preceding crash. Not so much in Japan. And soon after the "toy" angle was soon dropped, as were most of the more "gimmicky" control schemes (such as ROB and the Light Gun) when the core experience of simply the game and the default controller were found to be by far the most popular aspect of the system.
Again, to be fair, much of the features implemented to breeze through chunks of the game were not overtly easy to access for or known by the typical player, especially in the age before the internet. But you're right, for the most part the Mario games are sliding down a scale of relative ease (with exceptional levels here and there), though that's a bit unrelated to the point I'm making. It's true by making the games easier they are trying to appeal to a wider market, but they're not alienating the more "core" audience in this fashion, unlike obtuse control schemes which face derision with much more frequency.
And who knows, with the new wave of Dark Souls-esque game design, some games are upping the difficulty again. Or, like with Zelda, at least eliminating much of the previous hand-holding. Perhaps Mario, in one way or another, might follow suit. (Though given that he's Mr. Accessible, probably not)