This doesn't work when dealing with something unexpected, which to me is when fudging most often comes into play. In such a case, "you should have prepared for the unexpected occurrence" is not helpful.
Just to be clear: Disagreeing with your opinion doesn't mean that I don't think that you have a right to your opinion, or that I disrespect you as a person.
What you've said here, I concede, should allow the DM to "fudge" in a way: The DM should be able to fill in parts of the world "on the fly" based upon what he knows about the parts of the world already filled in.
For example, if you need to know the Orc Chief's name, or the City Guard's name, it is (in a way) "fudging" to make that information up. This is especially true as the "illusion of reality" of the milieu is best served by pretending that the information was already known.
Likewise, when the players head off into unknown (undeveloped) territory, the DM must not only decide what is there (preferably based upon the reality of the campaign milieu), but must behave as though it was already there, or the "illusion of reality" will suffer.
I don't think that there is anything wrong with making several small lairs that can be placed when needed to facilitate this type of problem; I know that there are people who think that this is too much "fudging". However, once "what is there" is decided, I don't think that the DM should backtrack.
I certainly don't think he should fudge rolls.
I think that the desire to fudge rolls stems, almost exclusively, from the idea that the "DM knows best" and/or "the DM determines the desired outcome". IMHO and IME, players don't want to know that the DM fudges not because "drama" is harmed, but because (specifically) the "illusion of consequence" is harmed.
I will certainly grant that damaging the "illusion of consequence" can and will damage any feeling of drama that is occurring. As I said upthread, I have firsthand experience of the same.
I also tend to think that the experience of drama over the long term is more important than the experience of drama over the short term. I.e., if you fudge now, the PC lives, and the short-term experience of drama is satisfied. But, because the player figures out that you are fudging, the long-term experience of drama (and therefore, the many future short-term experiences of drama) is hurt.
Worse, trust in the DM is hurt. Players start doing, as noted upthread, "silly things" to see just how far they can push it.
Maybe your Bluff skill is just waaaayyyyy higher than mine, but IME and IMHO, sooner or later players always figure out that the DM is fudging. Just as, sooner or later, players & DM always figure out if Joe Player is cheating on his die rolls.
RC