As long as it is to his benefit. After 12 years as a realty TV star who has more name recognition than any other candidate with the possible exception of Hillary (you know you are famous when you are referred to by only your first name). Millions of Americans think they know The Donald after being in their homes for 12 years. He is a celebrity.
I have only seen a few episodes of his show. My wife and daughter love it. But even I know when you hear, "You're fired," it means Trump. He is the Muhammad Ali of politics. An irrepressible self-promoter who backs up his brash and sometimes crude rhetoric with results.
If Trump were running for celebrity in chief bashing Megan Kelly might keep him in the limelight but the kind of celebrity one needs to be president is a bit different. The problem for Trump is he must stay in the limelight and to do that he must become ever more and more outrageous until he crosses more and more lines and alienates more and more people.
The more he bashes Megan Kelly the more he taints whatever legitimacy he had acquired for plain talk about hard issues. Now it's just gutter talk about personalities.