TheStreet: David Letterman’s “Late Show” has been a dependable source of income for CBS, but it’s also been losing viewers. As Letterman presents his final show, the network is pitching advertisers on the notion that his replacement, Stephen Colbert, can win over millennials. Though Letterman has been being showered with praise over the past month, young people have been mostly flocked to late-night rivals Jimmy Fallon on NBC Jimmy Kimmel on ABC.