If Rome can fall, so can “American Idol.” Last week at the New York upfronts — the TV networks’ equivalent of a high school pep rally, where executives swear that the upcoming fall schedule will change life as we know it — Fox dropped the bombshell that “Idol’s” upcoming 15th season will be its last.

From a ratings standpoint, it makes sense.

The juggernaut that once attracted 30 million fans a week has dropped dramatically in viewership and influence in recent years. You have more of a chance at stardom these days by posting a YouTube video from your bar mitzvah than by being cooed over by Jennifer Lopez.

Fox will certainly capitalize on its long farewell with appearances by past winners and judges, but it’s going to need more than Clay Aiken and Ruben Studdard duetting on “I Got You, Babe” to climb back into first place.

So what lies ahead for the four major networks? A quick evaluation:

Fox

Disappointing results for “Utopia” and “Sleepy Hollow” led to a 20 percent drop in viewers in the 18-49 demographic, which means that an “Empire” spinoff may be in the near future.

In the meantime, expect heavy promotion of the January return of “The X-Files” — a risky proposition, considering that a lot of younger viewers might wonder if that’s the title of Edward Snowden’s website.

More promising is Ryan Murphy’s “Scream Queens,” which sounds like a lighter version of “American Horror Story,” and a three-hour live production of “Grease” in late January.

As predicted, “The Mindy Project” didn’t make the cut, which wouldn’t be so terrible if it looked like Fox had some sort of savvy sitcom strategy. Comedies starring John Stamos, Rob Lowe and Fred Savage may be appealing — if you’re still using your Brat Pack lunchbox.

CBS

The network has said for months that it planned on picking up “Supergirl” — and I never quite believed it.

The drama, which will go up against “Gotham” on Monday nights, seems like an odd fit for a channel whose average viewer is 126 years old, but it sounds like the suits are confident about its prospects, especially if the franchise-friendly network can figure out a way to churn out “Supergirl: Las Vegas.”

CBS’ biggest surprise was the decision to hold back “2 Broke Girls” for midseason, which means telling inappropriate groaners for the short haul may become the responsibility of Charlie Rose.

ABC

Steady goes the course at the network that went from fourth to third last year with a 7 percent increase in the key demographic that advertisers salivate over.

ABC is standing behind “Fresh Off the Boat” and “Black-ish,” which is good news for those hungry to see more people of color on TV. Plus, there’s the return of one of TV’s most diverse casts: “The Muppets,” which will be paired with “Boat” on Tuesday nights.

Biggest disappointment: The cancellation of “Cristela,” a showcase for up ’n’ comer Cristela Alonzo, who never got the support or time slot to give her a legitimate chance.

NBC

Once the home of TV’s most sophisticated comedies, the network is putting on a more sober face by axing all four sitcoms it introduced last season — including the promising “Marry Me” — and adding nine dramas, including a reboot of “Heroes.”

The most intriguing new offering is a live weekly series, “Best Time Ever With Neil Patrick Harris,” a sign that NBC is willing to overlook Jay Leno’s doomed attempt at a prime-time variety show.

CW

The youth-oriented network is largely standing by its current schedule, which includes “The Flash” and “iZombie,” but it has said goodbye to “Hart of Dixie” and “The Messengers.” Neither will be missed.