Lisa Edelstein, Chris Meloni

If you believe all of those doomsday predictions, the world is set to end in 2012. But the broadcast networks are bracing for a different kind of apocalypse next year, as several prime-time staples appear ready to call it quits.

Aging favorites like House, CSI, Desperate Housewives, Grey's Anatomy and Law & Order: SVU are all nearing the end of their lifespans, with ratings declining and costs rising. Furthering these shows' slides are the seismic casting shifts that rocked the industry in recent weeks. CSI's Laurence Fishburne, SVU's Christopher Meloni and House's Lisa Edelstein all chose not to renew their contracts. And more stars — including House's Hugh Laurie and Grey's Anatomy's Patrick Dempsey and Ellen Pompeo — may be gone once their deals end next year.

"All four networks are coming to a pivotal point where franchise shows will start to dry up," says a rival cable executive. "Ratings are going down, costs are going up, and actors are getting ready to leave. And there's no obvious stuff coming to replace it just yet."

One network executive admits that he and his competitors are probably guilty of keeping programs on the air past their sell-by date. The shows start to suffer creatively, which sometimes damages their legacy. A declining series also takes up valuable shelf space that might otherwise be used to launch a new hit. "Generally there's two years when you agonize over killing a show," he says.

But when a series is still garnering decent ratings, it's hard to let it end. Programmers like using veteran shows to help launch new series. And when a well-established show ends, its replacement isn't likely to do as well. That's why Two and a Half Men and The Office are continuing, despite the losses of stars Charlie Sheen and Steve Carell.

At Fox, House is only back this fall because producers found a way to slash budgets (which resulted in the departure of Edelstein, whose salary was cut). Laurie was already signed for next season but has made it clear he's ready to toss his cane aside.

The news may be rosier for Desperate Housewives, because ABC is expected to face more ratings woes next year. ABC has picked up just one more year of the soap, but it will likely want to squeeze out another. (They have a deal with the actresses for two more years.)

Over at Grey's Anatomy, Dempsey and Pompeo have made plenty of noise that they're ready to exit after 2011-2012. But Grey's boasts an ensemble that can keep the drama alive even after its major characters depart (witness the longevity of ER). And like Housewives, ABC will probably still have a need for Grey's.

Meanwhile, CBS is looking to reduce its reliance on the CSI franchise. Just as NBC has whittled Law & Order's five editions to one (SVU), CBS is expected to cut at least one, and maybe two, of its aging CSI series. "I was surprised all three CSIs came back this year," says one rival.

Any of the three could go. CSI: NY is the lowest-rated, but CSI could be in danger now that Fishburne is out and Marg Helgenberger has reduced her appearances. Fishburne was said to be eager to get back to films, and CBS was likely fine with letting the expensive actor go. CBS will shift CSI from its longtime Thursday home to Wednesdays this fall. And as it enters its 12th season, CSI has lost half of its audience from its 2004-2005 peak. That's a sign that it might be time to call in the TV forensics team.

Network execs remain optimistic that the next smash is around the corner, and it's critical that they find a new generation of hits this fall. Says Horizon Media's Brad Adgate, "It's become a crucial year because last season's new hits were lackluster at best."

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