I decided to go back and read The Final Solution to see how much/little they used for the story. Interestingly, the thing that spurs Watson on to writing it is defamatory press coverage of Holmes, bigging up Moriarty (courtesy of his(one of his) brothers.)
Disappointingly, the story itself is little more than a sketch, Conan Doyle has Holmes constantly saying he'll bring down Moriarty and then that's it, no more mysteries. I'd forgotten (due to all the TV and movie versions that have fogged up/established -take your pick- the continuity) that Moriarty appears in this story as a master villain that Watson had never heard of, so neither had the readers up to this point. There's no foreshadowing, just: he's the villainous version of me, I'm taking him down.
Because Watson doesn't even see the battle on the Falls, we only have his speculation. So the fall of The World's Greatest Detective actually happens off-camera, so to speak.
Conan Doyle was bored with Holmes and it shows, he's just finding the simplest way of getting rid of him, and then doing it with shockingly little respect to his audience.
Both the Guy Ritchie movie and the Moffat/Gatiss show handle the story so much better than the character's creator.
