Why am I bringing this up? Because Star Fox 2 was never finished and it reminded me of how you said parts of Castlevania 2: Simon's Quest played like a demo, like parts of the game weren't really finished (the poor clues leading to having to kneel at that cliff, or that town that has no people in it.)
In spite of all my golden memories of SMW--again, because of it being my first game--I was surprised at how much of it I'd actually *forgotten*. I thought it was a pretty easy game but I actually got lost in some levels, stupidly--Vanilla Fortress?!?--and didn't remember everything I thought I would remember, which shocked me because I figured I could probably play any classic Mario game in my head, but this proved not to be entirely true. I died about 10-15 times in my playthrough, which is 10-15 more times than I thought I'd die. I forgot about things like the Forest Ghost House having two exits, or all the ways through that one Chocolate Island level which messes with you based on how quickly you're getting through it.
Parts of this game actually did play like a demo--crappy little "bonus areas" that have next to nothing in them besides a few coins, and levels like Donut Secret 2 which just suck. Star World's levels struck me as rather stupid, actually. The Special Zone is still pretty cool. Some of the doors you can enter in Bowser's Castle are pathetically easier than others.
I got the impression that maybe Nintendo kind of rushed bits and pieces of this one too. It would have come out just a year after SMB3 in America, but I guess that isn't the case in Japan where SMB3, I have to keep reminding myself, was available in 1988, or 1989 if you're a cast member of The Wizard. Think my theory has any credence?