Why Does Scifi TV Get A Seven Year Itch?

I had a thought while avoiding this week's three-hour series finale of medical drama er; why don't science fiction shows last fifteen seasons? Does all SF TV (Doctor Who aside) have a seven year limit?

Think about it; even the runaway successes don't make it past a seventh season. With the exception of the original and most recent series, all of the Star Trek series lasted seven seasons. Buffy lasted seven, as well, and BSG lasted four and a bit (The miniseries always feels a little like a season zero to me). The only three shows to break this rule that I can think of are Doctor Who (which ran for 26 years originally, then went on hiatus for 16 years before returning), Stargate SG1 (10 seasons) and Smallville (About to head into its ninth).

The practical answer, I'm sure, would involve actors wanted to stop playing starship captains and go off and do something else for the Hallmark Channel or guestspots on Leverage or whatever; seven years seems the limit on contracts for most actors aside from Tom Welling and Allison Mack. But what keeps SF shows from just swapping out actors and leads like Law & Order and continuing on regardless? Part of me wonders what the fan raction would've been had Star Trek: The Next Generation had introduced the cast of Deep Space Nine into the show around its sixth season and just continued with them as a new Enterprise crew for an eighth, and beyond, with the cast and crew of Voyager joining in at a later date. Could we have had fifteen years of Star Trek, or would fans have jumped ship because their favorite characters were gone?

You could make the argument that no show deserves to run 15 years no matter what the genre; certainly I'd admit to dropping out of er way before the ten year mark, never mind making it all the way to the end. It wasn't that I was bored of the cycling in and out of numerous characters who shared similar traits and ever-increasingly dramatic personal demons, but also that the stories themselves became repetitive and predictable. The same could be said of the final years of Star Trek: The Next Generation, Voyager and Stargate SG1 (and of recent seasons of Smallville and Doctor Who, for that matter). Even the last couple of years of Buffy and Deep Space Nine began to feel stale, as if the writers had told all the stories that they'd wanted to. Is there something about sci-fi drama that exhausts itself in its need to constantly up the scale and scope of its stories each and every year? Perhaps actors seeking greener pastures isn't the only reason why even successful SF ends around the seven year mark; maybe the creators run out of new ideas that they're able to create on a weekly television budget, as well (After all, Star Trek: First Contact was one of the best Next Generation stories despite coming years after the seventh season of the series).

Part of me wonders why we haven't really seen a successful sci-fi procedural set up so as to allow for characters to come and go more freely than a Star Trek, but also to take advantage of a syndication-friendly done-in-one format without the constant demands of an ever-growing internal mythology and backstory... A CSI: Mars, or whatever. The closest things I can think of to that would probably be The X-Files, which tried to replace its stars in its eight season (Hey, another show to add to my list of 7+ seasons! X-Files ran nine, of course) without much success... probably because of the crushing weight of the show's mythos being tied directly to the original leads, and SciFi's Eureka, although that seems to be creeping towards a "bigger picture" backstory ever so slowly. It seems like an obvious idea, considering the success of police and medical procedurals, but science fiction shows always seem to gravitate towards intricate backstories and centering the shows around the characters, instead of the plots, as some kind of cliched way of giving "regular" audiences something to hold onto amongst the technobabble. But, as Lost and Battlestar Galactica push SF TV towards a model of shorter, more novel-like approaches, it'd be nice to see Eureka or even SciFi's new Warehouse 13 demonstrate that SF TV can do something else, and have the longevity of more "mainstream" shows.