But this time it's different, Leno contends, offering another hard fact: The older generation has to make way for the younger one.
Britain's Queen Elizabeth II can keep 65-year-old Prince Charles cooling his heels. Leno doesn't have the power to do the same with Fallon, 39. The "Late Night" host is moving the show from its longtime Burbank home, near Johnny Carson Park and off Bob Hope Drive, to its New York birthplace when he debuts as host on Feb. 17.
"It's been a wonderful job but this is the right time to leave," said Leno, whose once-dark mop of hair is now a neatly groomed silver. "I'm at that age where I don't really listen to the (current) music anymore. I'm not a big tweet guy. A 63-year-old guy reading Miley Cyrus' tweets is a little creepy. Move on."
He makes the argument with the precision of one of his reliable monologue jokes, just as he did when he claimed to understand NBC's decision to evict him for O'Brien — even as he reamed the network on the air.
The years between then and now have seen changes come at a quickening pace, with an ever-more crowded late-night arena and a shifting media environment. Fallon's parody music bits with contemporaries like Justin Timberlake are perfect cut-and-pastes for sites like YouTube that drive young viewer attention and offer new potential for ad sales as network revenues shrink.
In 2012, "Tonight" laid off 20 staffers and Leno took a 10 percent pay cut. The show has averaged a 3.5 million nightly viewership in the past 12 months, which pales in comparison with the double-digit audiences it once claimed.
Leno is planning to expand the comedy club gigs he never abandoned and various outlets for his automotive passion, including the Web show "Jay Leno's Garage," and the magazine and newspaper pieces he writes. He insists his schedule won't include another late-night show, which could only be what he calls "Tonight Light."
"It's hard to re-create this moment. It's like the fighter coming back. You got to be world champion, so it's kind of silly," he said.
"Tonight," which launched in 1954, was shaped by original host Steve Allen and nurtured by successors Jack Paar and Carson. Following them represented the pinnacle for comedians, and it was the role Leno coveted and won upon Carson's 1992 retirement.
His first few months were marred by Leno's longtime manager Helen Kushnick, who, as his first "Tonight" executive producer, was blamed for instigating nasty guest booking wars and fired in what then was characterized as one of TV's biggest publicity nightmares.
Worse was to come, when NBC's "Tonight" host succession plan hatched in 2004 went awry. Leno, who stoically endured insults from Jimmy Kimmel and others who portrayed him as having stolen O'Brien's job, says the past is past. CBS' Letterman, who once jockeyed with Leno for Carson's throne, echoed that.
"How long can I carry this with me?" he told Howard Stern during a SiriusXM interview Friday. He spoke of calling Leno when his second "Tonight" departure was announced, their first conversation in several years, and tipped his hat to his rival when Stern asked if Leno sounded sad.
"I wouldn't say sad. There's nothing to be sad about. He's had a tremendous career there," Letterman said, graciously.
Others have chimed in. "Politically Incorrect" host Bill Maher, a regular "Tonight" guest, and Seth MacFarlane ("Family Guy," "Ted,") serenaded Leno last week to the tune of "Thanks for the Memories." It was reminiscent of Bette Midler's saucy "You Made Me Watch You" tribute to Carson. This parody was edgier.
"You've been retired and twice been fired for being No. 1. How stupid they are," the pair sang, zinging NBC as a surprised, bemused Leno watched.