One of the biggest problems with the massively popular BBC show Sherlock is well known by now, and also the reason why we're so long waiting for Season 4; it's the incredibly busy schedule of the lead actors: Benedict Cumberbatch (Sherlock Holmes) and Martin Freeman (John Watson).
In a recent Variety interview about his other smash hit show, Doctor Who, showrunner Steven Moffat discussed the format of the show and how it assures it's longevity, proven by the fact that it's been on air since the early 60s (the 16 years it went off air in the late 80s excluded). The beauty of Doctor Who's format is due to the fact that the main character changes his face and personality every few years, allowing for the titular actor to be replaced over and over when they move onto something else.
Moffat describes Doctor Who as "the all-time perfectly evolved television show", and it's certainly proving to be so as it's on its ninth season since the 2005 reboot (so that's nine NOT including the original series) and is showing no signs of slowing down.
Moffat: "It’s a television predator designed to survive any environment because you can replace absolutely everybody. Most shows you can’t do that with."
However, the same cannot be said for Sherlock. It's nobodies fault but the public's perhaps, as titular character actor Cumberbatch has experienced a dizzying rise to fame since the first season aired back in 2010. Despite the fact that it was renewed for Season 4 nearly a year and a half ago there's still little news about when production will begin for the next season, Moffat has confirmed that it's set to start "April-ish" of next year, presumably once Cumberbatch has wrapped on the currently filming Doctor Strange.
It's no secret that the large gap between seasons is due to the busy schedule of everyone involved, but as Cumberbatch and Freeman's careers are showing no sign of slowing down it's not looking good for the series longevity as Moffat himself has pointed out.
Moffat: "Once Benedict Cumberbatch gives up Sherlock, what are we going to do? We are going to stop, that’s what we are going to do. Most shows have a built-in mortality."
Unlike Doctor Who there's no replacing Cumberbatch as Sherlock. His portrayal of the socially dysfunctional detective is a large part of why the show is as popular as it is, and without him there is no show. The same can be said for Watson too; removing Freeman from the equation would crumble the core of the show, the narrative of which was born from their initial meeting.
It's not that Cumberbatch would necessarily want to leave the show, but for someone as popular and he is (not to mention a recent husband and father) juggling all those priorities gets difficult, especially now that he's involved in the Marvel Cinematic Universe as a character likely to become a big player in the films. But given the uphill struggle it's been just to get Season 4 up and running it's not looking great for the show in the long run.
Whatever happens let's hope that we get at least a few more seasons before putting the critically acclaimed show to bed, for now we'll be happy just with Sherlock Season 4, shame it's not due until 2017...