Reality TV is getting old period and talent shows especially are becoming tired. "The X-Factor" was recently canceled.
American Idol is still a top 10 show. Most networks would kill to have shows in the top 20 each week. Like any show that's been on for 13 years it's bound to lose it's original core audience. Some people blame the judges/bad contestants but the truth is everything that it "new" eventually becomes "old". Even Elvis Presley, Michael Jackson, and Frank Sinatra were passé before they died.
I'm amazed people still watch "The Bachelor", "Survivor", "The Real Housewives of (anywhere)", "Dancing With The Stars", "So You Think You Can Dance", "The Voice", "The Sing off", "America's Got Talent", "The Amazing Race", "Hells Kitchen", "The Taste", "Keeping up with the Kardashians" and on and on.
Having said that if a show manages to stick around for over 20 years it has a way of becoming an "American Institution" ala "The Tonight Show", "Saturday Night Live", "60 Minutes", "Wheel of Fortune", and "Jeopardy!" are a few examples of shows that have become staples in television. I can't imagine American Idol being around for 20 seasons but I guess no one thought those other shows would have lasted that long either.