What you neglect is Fox News is the only conservative voice among the major networks. The typical Trump supporter is in the age group of 45-65 and their primary source of new is TV and for conservatives, that means Fox News.
As Trump goes through the primary process and possibility a general election, he is going to need the very people, that is the establishment that he has waged a verbal battle with since his campaign began.
Few people seem to realize just how unpopularity Trump is with Democrats and independents. To overcome this, he's going to need a lot of allies that he just doesn't have. Antics to attract attention such as calling into news shows and asking to be put on the air, streams of outrageous tweets and facebook messages, and personal attacks are not going work in a general election. Exposure in the media is far less important in a general election than in a primary because the voters are already very familiar with the candidates. Voters are more concerned with issues than in primaries and they rely on the media to explain the candidates position.
Flopper they (Fox) need him more than he needs them. Fox News knows who butters their bread and CNN is just foaming at the mouth right now waiting to pounce. Trump has Fox by the balls and they both know it. In regards to Trumps unpopularity among Dems and Independents, you are sorely misjudging his crossover appeal.
How the hell did fox news ever survive before Donald Trump? Can we keep it real here please? CNN might need Trump, Fox? No they don't.
They survived because Americans devoured the propaganda and bullshit they were being fed. When Trump came around their base rallied around him only to tune in and see the bias BS Fox was pushing out. They were all left scratching their heads saying "WTF is going on here? No! We don't like Rubio! We don't want Bush!!" Fox is ******* tone deaf and millions see it. Yes, Fox needs Trump. Trump is viewers, and viewers are ratings, and ratings are money.....
Ratings have zero to do with a political debate. Ratings are for selling things.
Rump is a ratings magnet because he's a wacko, but a political debate is supposed to be about issues. If it isn't about issues, then it's worthless. Which makes it actually a positive thing that Rump won't be there to distract the issues away with his usual blusterfluff.
On the larger general point though, you are correct that Fox needs Rump (and any other controversy it can find) since at base it's not there to inform --- debates like this being the exception that proves the rule -- but to milk the consumer with scare stories.
But that's in the big picture, not really the instant case.