But there are new alternatives to Fox News online, and even on cable, that play to different parts of the conservative movement. Foremost is Breitbart News, which throughout last year took pot shots at Fox News when it viewed it as being anti-Trump, especially its former host Megyn Kelly.

Breitbart’s coverage showed that new alternatives were emerging as the conservative media moved into more complicated territory, along with its adherents. Breitbart, for instance, positioned itself as a purer “antiglobalist” alternative to Fox, where Mr. O’Reilly was as unpredictable as he was conservative-leaning and various contributors were solidly in the “Never Trump” camp. (On Wednesday Breitbart treated Mr. O’Reilly’s firing as a capitulation to the opposition, with a headline reading “Activist Left Gets Monster Scalp.”)

None of that is to diminish the dominance of Fox News, given its full cable distribution and the way its shows pepper the top ratings slots. That includes Sean Hannity, whose 10 p.m. program has become a central clearinghouse for pro-Trump talking points in a way Mr. O’Reilly’s never did.

Still, Mr. O’Reilly was in no small way responsible for the success of the rest of the prime-time schedule; he held what is known as a “tentpole” position at 8 p.m., where his ratings could feed into later shows.

Whether the new, hastily made schedule that Fox announced on Wednesday — with Tucker Carlson succeeding Mr. O’Reilly; the ensemble talk show “The Five” moving to 9 p.m.; and Mr. Hannity remaining at 10 p.m. — winds up being more permanent than temporary remains to be seen.

And while executives have been heartened by Mr. Carlson’s strong ratings since replacing Ms. Kelly, who has left to join NBC, he no longer has what was the best lead-in in cable news. (Last week, for instance, Mr. O’Reilly had an average weekday audience of nearly four million people.)