You can tell when you start watching Sherlock that Steven Moffat and Mark Gatiss were on to something. It just has that tone that seems to know it's special. There's the way it shoots London, the way it somehow brings Arthur Conan Doyle's material elegantly into a modern setting, and of course there's Benedict "this is the internet we have to mention him" Cumberbatch. All of these things exude a confidence that few other TV shows are lucky enough to afford right off the bat.
Just look at those opening titles! They just seem to subtly say "yeah... I'm here to stay, and you know it". In terms of what's getting brought back for another season, and what causes a stir whenever it returns, Sherlock has one of the most secure places on our screens.
Why then is this show so sheepish with the lengths of its series? Of course, British television is slightly notorious for having fewer episodes per season than American shows, but three episodes really is pushing it! Yes, they're each 90 minutes long, and no, the short stay Sherlock makes doesn't actually damage it's quality as a whole. But when a season ends after the exact amount of time it would take any other show just to get going, I can't help wondering how great Sherlock could be if it went all out and adopted a traditional 13 episode structure.
Is this simply a case of Steven Moffat messing with us? It's a win win situation for him, since he has to put less time into a show, and the fans will still be bristling with anticipation for another year no matter what. And if we complain about this, then who cares? Apparently that makes him a talented writer. Hey, Moffat! People being frustrated about your writing doesn't mean it's good!
So what could Sherlock Season 4 possibly lose if it adopts the model for a longer series? Well, a sense of prestige for one. It may not be apt, but we as an audience have developed the idea that a shorter season is just more worth while. A series with fewer episodes seems to promise it will look after the viewer, and won't run away from them. This is the difference between how a show like Game of Thrones is perceived against something like Arrow. Neither show is definitively better than the other (off the record, Game of Thrones is much better), yet one presents an air of worthiness, and mathematically has less chance to waste your time. Sherlock takes that logic to an extreme, giving the audience barely enough time to question if what they're watching is good or not. Obviously, it also helps that Sherlock is very good, but hey.
A longer series with shorter episodes would also make Season 4 detract from Conan Doyle's original books, which weren't so much come and go episodes as they were comprehensive stories in their own right. Bringing Sherlock closer to something like CSI would no doubt be an insult to fans of the character.
Imagine what could be done if Sherlock Season 4 could weave plot elements through multiple episodes, and built towards a logical finale that Moffat couldn't mess up because this isn't Doctor Who! Imagine if The Reichenbach Fall had more than just one episode building up to it, and was the culmination of numerous plot beats threaded through the series! Imagine how much more mind blowing that cliff hanger would be if twelve other episodes had been leading to it.
Sherlock seems comfortable with how it is, and switching to a 13 episode structure definitely isn't its key to its survival, but doing so might just be the best way to please a fandom that was so carefully cultivated. And if there's anything to be learned from the internet, it's that people like it when they get given more of the stuff they like. Those are just facts you can't argue with!