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.