Like with many things, fandom is only as good as the people joining in, whether actively or passively. The more people who are active, the better it is. More fic, more art, more vids, and recs, and discussions, (but we can't forget the audience is necessary too.)
I can understand drifting away from a fandom, feeling like you've said everything there is for you to say, no longer getting that oh! I need to write/vid/draw this! idea, finding other characters in other fandoms you want to read about, look at, and watch. The allure of the shiny shiny can be very strong.
But all too often what I see is that diminished drive and activity accompanied by --- hell, sometimes justified by --- plaintive cries of 'this fandom isn't what it used to be :('
I've seen a lot of different people say this about many different fandoms. I've done it myself.
Well, yes. If you're no longer writing that much fic/making vids/creating art/joining in discussions, chances are you're going to miss the excellent discussions and plot bunnying and general craziness and hilarity that fandom's notorious for. The general rule of fanfic is "if you write it, they will come". If you want to see a particular theme explored, write it, and see the inspiration it gives other writers. I'm fairly sure these maxims apply to all facets of fannish activity.
I always feel like it's a bit of a disservice to new fen when old timers say 'this fandom isn't like it used to be :('. No. It may not be. But it could still be rather excellent. It could be smaller, yes, less active, but no less wonderful.
I've lost several waves of friends in Life on Mars fandom over the years --- some of whom I'm still, thankfully, friends with! \o/ But some with whom I've lost touch. Do you know what? I made new friends among the new fen, by trying new fic, engaging in discussion, interacting. I wanted to do this, so I did. I've long, long, long since given up saying 'this isn't what it used to be :(', even though it isn't; I've seen the fandom take many incarnations and for, oh, a year there, the fandom was a hotbed of inspiration and fervour. But I --- I like a lot of the new people who have come over the years, and I think they've brought valuable insight, and they've created some truly great stuff. I don't want to discount them because they're new.
I have lost the drive to fic, mod or set up, before now. There have been some months where I haven't had the least interest in a time travelling cop. Quite a few times where I've said "this will probably be my last fic" (with the insistence that I am, always, a liar.) But I've lurked around until I felt ready to participate again. I eventually learned not to blame my lack of drive on new fen.
I'm not saying people shouldn't be allowed to drift away from a fandom, or that they must cling on even if it isn't enjoyable any more. That would be ridiculous. What I am saying is that 'this fandom isn't what it used to be :(' isn't the world's best excuse and I'd quite like not to see it any more.