His ratings aren't appreciably different from Letterman towards the end. However, Colbert hasn't ever come remotely close to the number of viewers Letterman had during his heyday.

In general, late night talk shows have been on a long, slow decline and aren't all that relevant as part of the national conversation any more.

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Talk shows and late night shows were once one of the few chances to hear from celebrities and assume they're "candid", also used to debut groundbreaking music. Now, they're fixtures for either the older or those who are super into celebrity culture.

Steven will never get even close. He does well enough that they want to keep him and let him do some big-budget things but Letterman dominated in an era where his kind of show was on the rise.

Colbert is a bigger recognizable star because of his comedy central show and politics than his talk show. Which is more about maintaining a particular audience.

Not an easy thing to map. It very easily can be asserted whether or not Letterman was a better Late Night host, but in todays landscape, TV ratings can't really be used to be compared to the past since across the board they're much much lower. Same with scripted TV, in it's heyday a show like CSI was able to pull over 30 million viewers. By the time it went off the air in 2015, it was pulling 7 million. It's successor series CSI Vegas hasn't even crossed 5 million since premiering in 2021.

…and they will survive with a smaller audience. For network TV nowadays it’s just more important to know your audience. A specific demographic will often be worth more to advertisers.

When Reddit’s least-favorite show (The Big Bang Theory) had its finale a few years ago, I compared the numbers. TBBT ended with a number that, despite it being a finale and getting a higher-than-normal rating, even that would only have been enough to give it a 50/50 shot of cancellation in the 90s.

Toward the end, Letterman was pulling in 2.8 million viewers. Colbert pulls in 1.8 million.

not close, but they can't really expect to be in the modern era