Ah, Conan Doyle . . . always the troublemaker!
His half-hearted attempts at continuity spawned an entire area of study just to try to right it. His tossed-off statement "You may marry him . . ." gave justification to all sorts of craziness. And then there's his attempt to kill the goose that was the golden egg, resulting in a climax that comes mid-way through the story cycle.
We have three major Sherlock-related franchises right now, all facing the Moriarty cliff. Not the Reichenbach fall, which "kills" Sherlock, time after time, but the drop that must inevitably follow when Sherlock Holmes has faced his greatest foe . . . and is done.
The Canon of Sherlock Holmes wasn't built as a sixty-story arc, being a pioneer in serial-storytelling with a continuous main character. It wasn't like Doyle sat down and went, "I'm quitting after sixty, so fifty-five through fifty-nine will be the run-up to Moriarty, then in sixty, Sherlock Holmes will face his greatest foe!" And given that Doyle seemingly didn't wait, it gives modern producers of Sherlock material ample excuse to rush to the good part. Guy Ritchie got to Moriarty in movie one, then took him to Reichenbach in movie two. Moffat and Gatiss did likewise with their three-episode series. Introduce Moriarty in series one, kill him in two.
Moriarty makes for a really great battle, yes, but it's kind of like having Superman fight General Zod in his first movie . . . where do you go after you've met your match? Go after somebody who's even greater than your arch-enemy? The whole point of an arch-enemy is that he's your greatest enemy, the top of the line, the big challenge. Sherlock Holmes spends the rest of the stories missing Moriarty, because he knows he's never going to get that challenge again.
Can either the Ritchie or Moffat/Gatiss franchises sustain an ongoing series, even if their cast is all agreeable and doesn't go anywhere, once they've jumped off the Moriarty cliff? Other ongoing characters get additions to their lore over the years. (Take Superman and General Zod, for instance. That guy came very late to the party, after decades of Lex Luthor.) Will Sherlock Holmes eventually get an arch-nemesis greater than Moriary, simply given the fact that some really talented creator will want to keep going after killing the professor? Will Holmes get a Zod? (Or a Doomsday, if you really want to geek out . . .)
American television, of course, being the medium of drawing things out as long as possible, transformed Moriarty into a Catwoman figure, who's too much in love with her nemesis to kill him. And if she's not trying to kill him, he can't really kill her in self-defense. So off to prison she goes, to bedevil him another day, and, really, not truly filling the arch-nemesis role. If your arch-foe isn't coming at you with a full one hundred percent effort, well, you really need to pick a new villain to add the "arch" to. By turning Moriarty into a sort-of Catwoman and pulling her teeth, Elementary kind of winds up at the same Moriarty cliff as if she died . . . needing someone who can really come at their hero with villainous guns a'blazing. (When it's your ex, there winds up being all sorts of relationship overtones to mortal combat that just spoil the fun.)
Of course, modern cinema has shown us a whole new go-to plan for such things: the reboot. If Holmes never gets a better foe than Moriarty, he can just keep rebooting every few years with a new cast and a new start, and repeat the cycle all over again. But we're never getting to a whole lot of other great Holmes villains with that plan . . . Grimesby Roylott, anyone? . . . Baron Gruner? (And I'm not counting any "adaptation" that slaps one of those names on some serial killing kid working at a McDonald's and calls it done.)
The Moriarty cliff is a big one, and unlike Sherlock Holmes at Reichenbach, we're not sure we're going to see a survivor at this point. I'm looking forward to seeing if we get one.