Only children believe that Paul Daniels actually cut's the lovely Debbie McGee in half because these acts are being done with respect to a well known formula. We no more believe in the magic of Paul Daniels than we believe that Arnold Schwarzenegger literally IS the Terminator.
I do have a lot of sympathy for the frustration people feel about Derren's act too, and in fact just a couple of days ago I had a very long conversation with someone at a wedding who kept insisting that he had studied Derren's techniques and could do "all of that NLP stuff"
So let's say a magician is doing the cup and balls act and his explanation for the trick is that the balls are being moved by invisible fairies, as you say only a child would believe him. Now what if the magician does the same act, but this time claims that he is using "ancient wiccan magic" now not only will children believe him, but also a lot of teenagers (and maybe some adults too)
So where do you draw then line, can you only do a magic trick if > 80% of people can see through it?
Derren is the only magician I can remember having ANY audience at all in the U.K. since I was a child, so his choice seems to be.
A) perform a trick that convinces more or less no one and have everyone say "oh he's just another boring magician"
B) Perform a trick that actually convinces some people it's real and have some people say "He's a con artist"
As far as Paul Daniels doing tricks to a well known formula, well so is Derren, and some of the tricks he performs are literally "the oldest tricks in the book" and the only thing that separates him from Daniels at all is the explanations he gives for them.
The only place I can see where a line can be drawn is how the performer represents themselves after the act has finished, which with Derren isn't where a lot of people think it is.