I think (from the marketing point of view) Nintendo did the right thing. Most of the people bought the game blind because it was the new Mario game and because the first one was just a blast.

Even if the game was very different than the first one it was still a good game. First I was a bit disappointed after playing it (and it is still my least favourite Mario game from the NES and S-NES) but I got used to it. And so did most of my friends. I don't know if the original game would have been so successfull without the changing into the Mario franchise. It would have sure got less attention and all. Plus I think most of the gamers would not have bought a new Mario game that is similar to the first one but only harder. Mainly because most of the gamers (at least in my area) were kids and the parents had to buy the games. So why buy a game you allready have when you can get a new one? As I said at the beginning: Nintendo did the right thing.