Those feeling waves of nostalgia over the departure of “American Idol” on April 7 should probably wipe away any tears. A concept that shines this brightly won’t be gone forever – or maybe even for very long, as a few of the principals have already suggested.

But when (not if) it does return, it won’t be as big. Those moments in the cultural zeitgeist tend to be singular, and not quite so easily replicated as simply digging the title, theme music and set out of storage.

The story of this century in network television, really, has centered on the reality-TV mega-hit, beginning (cheating on the time frame just a bit) in 1999 with “Who Wants to Be a Millionaire,” followed by “Survivor,” “Idol,” “The Apprentice” and currently “The Voice,” with less durable commodities (10 points if you said “Joe Millionaire” or “Temptation Island”) emerging along the way. While cable has steadily risen as a creative force on multiple fronts, the broadcast networks have relied on eruptions from unscripted TV (the latest being NBC’s “Little Big Shots”) to buoy their numbers and occasionally engineer a rapid turnaround.

All those shows, however, come with a certain shelf life, a moment where the torch isn’t doused, necessarily, but no longer smokes the competition. In the case of “Idol,” that flickering can be directly traced to the departure of judge Simon Cowell, who announced his plans in 2010, even if it took a while for the damage from his exit to fully manifest itself.

Cowell, in fact, delivered a double whammy, not just leaving “Idol,” but convincing Fox to pick up a lookalike competition show, “The X Factor,” on which he’d appear. “There was no way we were ever going to lose Mr. Cowell,” Fox’s then-reality chief, Mike Darnell, said at the time regarding the prospect of letting the new series land on a rival. “We were never going to let that happen.”

Yet after resisting the temptation to do more than one edition of “American Idol” a year and risk diluting its ratings, Fox did exactly that, just under a different name. And while the suspense over who would replace him kept the existing show afloat for a time, interest began to diminish, perhaps a tad faster than it otherwise might have.

In this case, Gordon Gekko got it wrong: Greed was bad. Or at least, bad for “Idol’s” longevity.

For all that, Fox didn’t really have to cancel “Idol,” even in its current watered-down form, but is probably wise to do so. As we have seen time and again with a flurry of reboots, there’s nothing like a bit of absence to make the heart grow fonder. In addition, it’s not like the producers have to go convince a couple of former-TV-actors-turned-movie stars to reprise their roles. The great news about reality shows is the raw material usually comes pretty cheaply, and almost everyone (including a host like Ryan Seacrest) is replaceable, even if what Cowell brought to the table turned out to be an enormous asset.

Another truth is that as the video market continues to fragment, and more players squeeze (or stream) into the game, each new hit seems to assemble fewer loyalists than the last one. Sure, “The Voice” does swell, but it’s a far cry from the more than 30 million people who dutifully showed up for “Millionaire” or “Idol” in their heydays, or the staggering 52 million that watched the first-season finale of “Survivor.”

These shows can still deliver a water-cooler-type experience – where millions of people simultaneously tune in and tweet up a social-media storm – but to paraphrase “Sunset Blvd.’s” Norma Desmond, with each new hit, the pictures get a bit smaller.

So rest assured, we haven’t seen the last of “American Idol.” But when it comes to hitting those operatic high notes ratings-wise, its best days are almost certainly behind it.