After 21 years, David Letterman announced today that he will retire from “The Late Show with David Letterman” in 2015. That is a remarkable tenure. But looking back at the state of late night television when Letterman moved to CBS in 1993, it is extraordinary how little has changed.

FILE - APRIL 3: David Letterman announced on his show today that he will retire in 2015 NEW YORK, NY - MARCH 26: David Letterman speaks onstage at the First Annual Comedy Awards at Hammerstein Ballroom on March 26, 2011 in New York City. (Photo by Dimitrios Kambouris/Getty Images) David Letterman speaks onstage at the First Annual Comedy Awards at Hammerstein Ballroom on March 26, 2011 in New York City. (Photo by Dimitrios Kambouris/Getty Images)

Yes, segments on shows like “Late Night With Jimmy Fallon” have been precision-engineered to go viral online, and the jokes and the celebrities are different. But the men behind the desks looked awfully similar. Jay Leno, who took over NBC’s “The Tonight Show” in 1992 may have retired (again) earlier in 2014, but his name still comes up whenever it seems like a network might be eager to revamp its late-night lineup. Conan O’Brien, who took over NBC’s “Late Night” from Letterman in 1993, now hosts his titular show on TBS. MTV got into the late-night game that year with “The Jon Stewart Show.” Fox tried out “The Chevy Chase Show,” which lasted only five weeks. Today, Chase’s fellow “Saturday Night Live” alumnus Seth Meyers is showing more staying power with “Late Night With Seth Meyers.” Arsenio Hall’s talk show faltered in 1994, but it was revived last fall.

As NPR’s Linda Holmes put it in a piece about the Leno-to-Fallon handoff at NBC last spring, “It would be great if it were a story about fresh versus stale, about who will control vast swaths of the landscape, about who’s better. But it’s not.” Late night television is a reliable part of the business model in an industry that is increasingly confused about how to make anything pop in prime time. And sticking with the same thing that has worked in the past tends to mean sticking with genial, middle-aged white guys.

For that reason, the more significant late-night announcement this week seems to be the news that Chelsea Handler is leaving her show on E!. Handler’s brand of humor is not exactly my shot of hard liquor. (She’s a vodka gal, I like bourbon.) But her departure leaves late-night television without a female host. The cancellation of George Lopez’s “Lopez Tonight” on TBS, W. Kamau Bell’s “Totally Biased” on FXX and T.J. Holmes’s “Don’t Sleep!” on BET have thinned the ranks of comedians of color. Andy Cohen, who hosts “Watch What Happens Live” on Bravo, where he mashes up recaps and drinking games, is the lone gay man in late-night. And I guess Chris Hardwick’s recap shows on AMC and “@midnight,” his comedy series, count for nerd representation.

It is hard to blame networks — which after all, are businesses, not creative retreats — for making use of talents like Fallon and Meyers, who have extensive training in their comedy pipelines. And Fallon’s recruitment of The Roots to be his house band, first on “Late Night” and now on “The Tonight Show,” has been welcome, as has the diversity of “Daily Show” correspondents like Aasif Mandvi and Jessica Williams. But the pace of innovation still feels frustratingly glacial. In the leadup to Letterman’s departure, we might do well to think hard about the fact that late-night’s past and its future look so strikingly similar.

Gallery: David Letterman to retire in 2015