Because when nobody else is talking about Sherlock Holmes, we still are. And when nobody else is making fun content about 221B Baker Street, we make our own.
Being a bit behind the curve, I have yet to ingest Mattias Bostrom's well-reviewed From Holmes to Sherlock, but it sounds like one could use it to chart all the peaks and valleys the legend of Sherlock Holmes has seen over the three separate centuries of its existence. And if one looked at that chart and pointed to the valleys, we'd all know who lives down there: Sherlockians.
When everyone else has left the theater, we'll sneak up on to that stage and do our thing in front of a house that only has a handful of diehards still in the front row. We'll know all the diehards by name very soon after that, and see very familiar faces climbing up to the stage again and again, while the general public goes about their business on the street outside until the theater of Sherlock again gets a hit during the Saturday night features.
There are other shows to wander off to, other fandoms that will take in those who need a heartier diet of fresh produce (he wrote, as he shifted metaphors), but for those that remain, living off stuff canned in earlier days, we'll get to see just how creative our chefs can be in helping keep our palates from getting too numb from the same, familiar tastes.
It's hard to say what the future will bring for Sherlock Holmes. Entertainment culture is anxious to squeeze every dime from any existing property, and Holmes's place in the public domain makes him a tempting well to go back to. We will definitely see adaptations and new attempts that fail to win over either Sherlockians or the general public -- oh, how we've seen those before, so many, many times. But we'll find our fun, and fun will be had.
And if it's back to just us, well, we've got a lot of new kids since the last drought (but do we have them, or do they have us?), and they seem like a whole lot of fun. I think we'll make it.