It is even less sustainable when you factor in the political leanings meaning he was missing out with the overwhelming majority of right wing boomers.
I think that the show was made that way by design by the network because they believed that it would lock in a demographic of viewers - and it largely did that. It would certainly be a recipe for disaster in the old model, where you needed larger viewership. With everything so fragmented now, you don't need that level of bi-partisanship. We can see if with news sources as well, Fox and CNN are fairly successful catering to one political bend. I personally don't like how political everything has become, and believe that it was a mistake. I watched Carson and Leno and did not know their politics because they were fair. Letterman, it was clear, was a Democrat but he had run ins with very liberal people on the show like Madonna and others and he seemed to float towards the middle-left. I would point out though that Letterman never had the market share that Leno had either, and that was likely part of it.
I meant mean-spirited in terms of the broadcast--Carson may have been a complete ******* off the air, but on the air he was 'reasonable' as you say. Someone else used the term "partisan," that might have been a better choice than "mean-spirited," but I do believe that some of Colbert's actual on-air broadcasts as regards Trump were mean-spirited. Fallon manages to criticize/needle/mock Trump through generally friendly satire and comedy. Colbert has gone off the rails numerous times and as a result I think he's lost much of his audience coming from the other side of aisle. All the late-night shows are partisan now, it's a big reason they've lost influence, audience share, and money.
