(CNN) -- There was a time when "American Idol" was unstoppable in the ratings.
For the past several years, Tuesday and Wednesday nights have been destination television for millions as they watched and voted for their favorite contestant.
The formula was fairly simple: Take a host of hopefuls, add in a goofy, yet nurturing judge (Paula Abdul), a slang-spewing, middle-of-the-road one (Randy Jackson), a tyrant (Simon Cowell), and a pearly toothed host (Ryan Seacrest) and watch the Nielsen numbers bubble -- sometimes more than 30 million viewers an episode, unheard-of numbers in these thinly sliced audience days.
But this season, the show's ninth, things have changed. Abdul is gone and the buzz continues to build that Cowell is soon to follow. The ratings, after peaking in the sixth season, have declined.
Just one season after many fans expressed their displeasure at the addition of fourth judge Kara DioGuardi, the announcement that comedienne/talk show host Ellen DeGeneres would take over Abdul's seat was met with less than overwhelming enthusiasm.
Could it be that the reign of "Idol" may be drawing to a close?
"Every show has a life cycle," said Chicago Tribune TV critic Maureen "Mo" Ryan. "I think ['Idol'] has reached that point in its maturity where it's not necessarily guaranteed to end up on the top of the ratings heap every single night, every single week, every single year. I think it will certainly be high up there, but I don't think it will be as huge a phenomenon as it has been."
The show still has a hardcore fan base, of course.
Dave Della Terza, creator of the Web site Vote for the Worst, where viewers are encouraged to cast their ballots for some of the more "interesting" contestants, said many fans get completely wrapped up in the show.
"People get really invested in it," he said. "We chronicle a lot of this on the Web site in that 'American Idol' has these crazy, rabid fans who will do anything for their favorite contestant and I think that's why [the fans] come back every season, because of that personal investment."
Yet over the years, he said, "Idol" fever has cooled a bit, as evidenced by the lack of superstar status of many of the recent winners.
"The last big star the show created was Chris Daughtry," Della Terza said, referring to the rocker who placed fourth on season five on the show and went on to sell millions of records and be nominated for a Grammy. "At this point, I think people are watching and realizing that it's not as exciting as it once was because the people who win don't necessarily go on to have these great careers."
Critic Ryan said the softening of the show's ratings is not surprising, given the different viewing options available to television audiences. And even on its worst day, most TV executives "would give a body part to have 'Idol's' ratings," she said. (Fox's competition has referred to the show as the "death star.")
But this season may prove to be an interesting one, given that Abdul's departure marks the first time an original judge has bid goodbye. Ryan also said if Cowell ends up leaving, it could prove disastrous.
"If this is Simon's last season then it is definitely going to be the last time we see 'Idol' reach its level of hugeness," Ryan said. "For me, that would be the tipping point toward 'American Idol' being a very, very popular and important show, but not necessarily the show that slays every other show out there."
"American Idol" runs on personalities, and producers are banking on Ellen DeGeneres to fill the void left by Abdul. "Her love of music and understanding of the American public will bring a unique human touch to our judging panel," "Idol" producer Simon Fuller said in a statement when DeGeneres was announced.
Devoted "Idol" watcher Kerri Lynn Pleasant said the addition of DeGeneres to the show is generating plenty of excitement for her as a fan, despite the many negative comments she has heard from other viewers who question the choice.
"[DeGeneres] knows stage presence," said Pleasant, a teacher who added that she will miss Abdul as a judge. "She may not have the music background, but she's got the entertainment background and I think that's going to help a lot on 'Idol.' "
Cheryl A. Young is also rooting for DeGeneres on the show despite the naysayers. Young taught season two runner-up Clay Aiken when he was a student at the University of North Carolina at Charlotte.
"She's funny and she knows show business," Young said. "She loves music and I think she will definitely be able to recognize talent."
If the adage that any publicity is good publicity holds true, even the debate over DeGeneres' suitability for the show might help draw in curious viewers.
Bob Payne, the editor for "Idol Confessions," Seattletimes.com's "American Idol" blog, said the show's producers have done well to exploit buzz. Last season, contestant Adam Lambert, who finished second, generated wildly differing opinions thanks to his "manscara" makeup and dramatic performances.
Payne said "Idol" occupies a unique position in television, and can always draw on that strength.
"It has a wide fan base demographic," Payne said. "There aren't so many shows that have such a wide age range in who can watch it, from kids to older people, and I think that's kind of the secret of its success."