I suspect, if
Firefly had not been cancelled, it would still have generated a cult following of some size - shows like that just
do, look at
BSG or
Game of Thrones, but would it be as famous as it is, or hailed as a near-perfect show by it's devout fans? I doubt it. We'd have threads going about whether
Firefly jumped the shark in season 5 or 6, and how lame it was to introduce Zoe's little sister without explanation half way through. No show can sustain an unblemished record through multiple years of production. Or, to put it another way...
But you can't always plan out a show in advance. In fact I'd go as far to say that it's a bad idea (excluding a few exceptions like Babylon 5). Before you've even cast your show or seen how things work out on screen, you're supposed to have plotted the entire series arc? You need to get comfortable writing for the show and you need to know your actors' abilities before planning out a huge arc that will last for years.
...even the best plan rarely survives contact with the enemy.
You're right, of course. I'm just a long running fan of 'arc' shows and nothing frustrates me more than shows which sell themselves as being that when they really aren't -
24 being the best example, but there are plenty of others making it up as they go along.
CSI and its ilk - different ball game - they never set out to be anything other than self contained, episodic entertainment ,with a little ongoing character development to satisfy long term viewers. It is the shows that set out to be epic operas clearly without planning their story
even a little bit that irritate me. Writers probably don't want to 'limit themselves creatively', but with weekly deadlines this quickly becomes not giving themselves enough
time to be creative.