When David Letterman does his final Top 10 next week it will signal the end of the longest late-night hosting career in U.S. TV network history and the end of an era.

The 68-year-old whose acerbic wit once made him a renegade in the late-night world announced last year that he was hanging up his hat after 22 years behind the Late Show desk.

His retirement, which begins after his last show on May 20, turns the page on a three-decade career.

The late-night TV landscape was a very different place in the 1970s when the gap-toothed comedian got his big break on The Tonight Show with Johnny Carson.

When NBC's Late Night wth David Letterman debuted in 1982, Letterman earned his reputation as the rebellious kid on the comedy block with his absurdist, cynical and pencil-throwing style of humour.

Sixteen Emmys and 30-odd years later, Letterman's shtick is showing some wear. When comedic contemporary Jay Leno announced he was throwing in the towel at The Tonight Show after 22 years, Letterman knew it was time to call it quits.

"If you look around at the other people [hosting late night] and look at me, it's almost like a pair of shoes you haven't worn in a hundred years," Letterman told Rolling Stone magazine.

"I still enjoy what I'm doing," he said. "But I think what I'm doing is not what you want at 11:30 anymore."

The new kings of late night, Jimmy Fallon and Jimmy Kimmel, have completely reinvented the late-night game, focusing more on creating viral videos than on sit-down interviews or written jokes.

But they owe the success of some of that reinvention to the pioneering Letterman, who originated many of their bits, according to bestselling New York Times writer Bill Carter, who penned two books on the topic: The Late Shift and The War for Late Night.

David Letterman sits at his desk on the television talk show Late Night with David Letterman, in New York in 1986. (NBC Television/Getty Images) "Letterman did a lot of this stuff," Carter said in an interview with CBC. "Jimmy Fallon loves to do games with the guests ... Well, Letterman did stuff like that. He had them do elevator races in 30 Rock hallways. He interviewed guests in barber chairs instead of regular chairs."

Despite being the trailblazer in innovative interviews, Carter said Letterman lost his edge because he simply stopped performing.

"He was outplayed because he stopped playing," said Carter, who says Letterman didn't like leaving the studio to record remotes for the show.

Letterman's viral weakness

Even Letterman himself admits that he's not up to compete in world of YouTube dominance.

"I hear about things going viral, and I think, 'How do you do that?'" he told Rolling Stone. "I think I'm the blockage in the plumbing."

Stephen Colbert, who takes over for Letterman on Sept. 8, will have to simultaneously fill the icon's shoes and bring new energy to the Late Show.

The 50-year-old, who is best known for Comedy Central's satirical news show The Colbert Report, will also have to play the YouTube game, while finding his niche in a pretty full playground.

Incoming Tonight Show host Jimmy Fallon, left, talks with outgoing host Jay Leno on Feb. 6, 2014. (Chris Haston/NBC/Associated Press) Competing with Colbert and the Jimmys are NBC's Late Night with Seth Meyers and CBS's The Late Late Show with James Corden.

Norm Sousa, who is the host of the only Canadian late-night comedy show Too Much Information, said the viral video phenomenon can't last.

"We are talking about seven or eight late night shows trying to have two to three viral videos a week," said Sousa, in an interview with CBC. "If you do the math, I just don't think it's sustainable."

The competition has created what Sousa calls a "late-night renaissance," but the the comedic battle royale won't be televised, he said.

"It'll be interesting to see how it will all pan out," he said. "[But] I don't think that late-night comedy will survive on TV."

Instead, he said, internet streaming services like Neflix and Hulu will inevitably get the last laugh.