Agree with almost all of this.
Late night like Johnny, Letterman etc were build on a foundation of 3 items.
1) Endorsement of upcoming movies via interviews
2) Various Skits by the host.
3) Guest appearances which were 95% comprised of comics and musicians. Yes, there were rare occasions where the Animal person or others would come on but mostly it was bands or comics being promoted.
Today #1 is so grossly over covered by social media, main stream media and other that it has become completely superfluous. People would watch to "get to know" the actors and then see a trailer of the upcoming film. People all have a "direct relationship" with their favorite stars these days via social media and have almost always already seen a trailer long before they appear on a late night talk show. Nothing new is occurring unless it is olds (over 60) who don't really use social media.
On Skits and stand up comedians/bands or #2 and #3 there are truly endless skit type and music promotion options on YouTube, social media, streaming services and main stream. I find far more new comedic and music acts through my carefully curated and extremely limited social media. I pretty much only like and follow artist, musicians and comedians. If they go to extensive political post I unfollow them. Not that I object to them having a political stance at all, I'm just not interested in theirs. I'm only interested in their art.
On top of all that, someone noted it upstream that if you want the highlights of Colbert you can get them on social media/YouTube without having to sit through all the filler, so why tune in?
The point of all that is at this point there is zero need or value of late night talk shows in today's digital media environment for 95% of the population. More unique shows like Comedians in Cars, John Mulaney Presents and My Next Guest fills a niche of getting to know celebrities in a seamingly more genuine less promotional method. They have a bit of skit but not a ton.
Net is there are SO many different sources to get the 3 components of late night that it's time has passed. It's over. Go home.