Well... More of Tolkien than of Sherlock, but I have watched and enjoyed Sherlock and read a fic now and then. Also like Doctor Who (which I need to catch up on), Narnia (childhood nostalgia's a huge factor there), Babylon 5 (which I'm overdue to rewatch), Star Wars, some interest in Star Trek, am on my first run through Buffy+Angel, like some anime, and like romping through random fantasy novels in the local library... So I've kinda got a spectrum of fandoms going.
I see great, I'm too a fan of Doctor Who and Narnia! Even though like you, I need to catch up with DW.. and I used to be a huge Narnia fan when I was younger. Back then I read tons and tons of fantasy novels.. now my tastes have changed a little, or better, broadened but my adage is "once in a fandom, always in a fandom" haha
...Well, I'm not sure if I'd go THAT far. Once in awhile I run into something that I used to like but now the writing style grates on me, or there's fandom drama on the internet that burns me out a little but on the affiliated community. But I at least tend to keep a soft spot for former-fandoms if they become former. ^_^ One've my friends burns through fandoms at a scary rate though - every couple of years she seems to fall in love with something new, dig her claws in deep, and then as soon as she comes up for air it's on to a new media love.
Haha, well, I fall out of fandoms too, but I can't let go of anything completely. I do not like things, I genuinely cannot do it, I OBSESS over them. Hard. But maybe you're right, what remains after all the burning passion has faded, it's just a sweet devotion ❤️