"If you don't," he said, "I promise you the network will just put another comedy show in this time slot. Then you'll be sorry."
It was one last dig in retaliation for his show's most harrowing episode, when ABC executives in 2002 secretly courted David Letterman to replace "Nightline." Letterman decided to stay at CBS and the attempt blew up in management's face.
"Nightline" will continue Monday with a revamped format and hosts Martin Bashir, Cynthia McFadden and Terry Moran.
Since March 1980, Koppel offered a serious alternative to late-night laughs as the only anchor "Nightline" has known.
For his finale, Koppel looked back at one of his favorite interviews: his 1995 conversation with terminally ill college professor Morrie Schwartz, which led to Mitch Albom's best-selling book "Tuesdays With Morrie."
Fortified by a double mocha, Koppel didn't sit down to write his closing remarks until Tuesday afternoon. His final show was taped several days in advance except for the ending, taped Tuesday evening.
He told viewers that he often quizzed "Nightline" interns on whether they knew old network anchors like Eric Severeid, Howard K. Smith or Frank Reynolds. He'd get blank stares in response.
"Trust me," he said. "The transition from one anchor to another is not that big a deal. Cronkite begat Rather, Chancellor begat Brokaw, Reynolds begat Jennings. And each of them did a pretty fair job in his own right."
He more directly thanked viewers for their loyalty in an e-mail sent earlier in the day to "Nightline" fans.
"It's been a joy and a privilege to occupy this chair for the past 26 years," he wrote. "I understand how many people grit their teeth on the way to work every morning. To have had more than 42 years now of almost always being able to go to work with a sense of excitement and anticipation makes me among the most fortunate of people."
"Nightline" officially began in March 1980. After Koppel, a veteran ABC diplomatic correspondent, spent several months briefing viewers each night about the Iranian hostage crisis, ABC put him in that time slot permanently.
Koppel's live interviews were the early drawing card. At the time, with CNN just starting as the only all-news network, it was a novel idea to bring interview subjects together from all over the world.
His voice rarely rose — and the famous helmet of hair stayed in place — but Koppel's incisive interviews continued through Hurricane Katrina and his memorable takedown of former Federal Emergency Management Director Michael Brown.
"Our legacy," Koppel told The Associated Press, "is that a serious news broadcast can be successful on all counts, without catering to anyone's baser instincts. `Nightline' has made a lot of money. It has been successful in terms of viewership, awards and accolades. But most important to me, it's been successful in not ever having to lower its standards."
In later years, "Nightline" evolved into a home for some of broadcast's most serious news documentaries, with each evening's show focused on a single topic.
Koppel, 65, slowed down in his last few years, often working three nights a week and, like the late-night comedians, taping his show a few hours before broadcast.
ABC will go live again with "Nightline" when Bashir and McFadden work from the network's Times Square studio in New York. New producer James Goldston said the spiffed-up "Nightline" will tackle several topics a night.
The Washington studio set where Koppel held forth is being abandoned, with a new one under construction for Moran, who will be based in the same bureau.
Those are big changes, but Goldston said he's extremely conscious of not scaring away the loyal but shrinking "Nightline" audience — its nightly average of 3.6 million viewers is down from 5.5 million a decade ago. Goldston promised several stories on the Iraq war and a series on AIDS in India in his first two weeks.
Koppel is not retiring — he will continue working with his producer, Tom Bettag. They were negotiating with HBO about doing documentaries.