Live network TV shows are the platform that is dead, and Colbert is a relic that became expandable, as late night shows have long been nose-diving in viewership -- really well before the streaming revoltion. But now, its much worse even, as Paramount (CBS's parent company) seeks to transition its resources towards its streaming platforms. SNL died decades ago, Late Night shows died about 15-20 years ago, and traditional evening news has been DOA for at least a decade.
Nobody needs or watches networks live for anyhting anymore, everything is going towards streaming platforms. Colbert's contract, plus his criticism of his employer, made it simply not worth the trouble to keep him around. Colbert's controversy with Trump and CBS's capitulation to his demands were simply the excuse needed for CBS to cut off dead wood. Nobody's picking up Colbert, if he wants to have success, he's gonna have to find it as a streamer on YouTube. Welcome to the future, and the present!