After being forced out of action a month ago due to testing positive for Covid, Stephen is out for at least a week due to a ruptured appendix, which was removed over the Thanksgiving holiday. That’s two weeks off the air just since the writer’s strike ended.
These days, it seems the networks are better prepared with alternatives than they were back in Johnny’s day. Replacement hosts tend to only be used for either planned or prolonged absenses.
Even most of Johnny’s guest host usage was of the planned variety. His contract eventually was changed to where he only did the show for 2/3 of the year, and only 3 shows a week.
Nowadays they’d probably run a different show in the slot. But, back then, the Tonight Show was so popular, they didn’t want to do that.
If Colbert were to be out more than a week, I could see them bringing in a guest host, especially after the writers’ strike. (I actually don’t know what they did during that time during the strike). But it would be a lot of logistical work to set up if he’s going to be back so soon.
They ran re-runs, as did the other late-night talk shows which had WGA writers; my understanding is that Fox News’ “Gutfeld!” ran new episodes during the strike, as their writers weren’t WGA members.