Late night TV isn’t dying. Like many things it is just failing to adapt.
Advertising is pulling out because nobody watched the show LIVE not because nobody watches the show. The vast majority of viewers watch on YouTube the next day.
It’s the same thing over again, Napster showed what people wanted and we got iTunes instead. Torrenting showed what people wanted and we got Netflix instead. People want to watch Colbert but as always the industry says “my way or the highway”.