There is a risk here for Trump. If (when) he starts going after Bernie, he will alienate a lot of people who love Bernie that may cross over to vote for Trump. If he debates Bernie, Trump should act Presidentjal and destroy Bernies arguments of capitalism over socialism without the personal attacks. It’s a tightrope for him but a tremendous payoff if it works.
Promoting the debate was brilliant because Hillary Clinton surrogates and went all crazy against it. Trump just said he is not doing it because it is not proper because Sanders is not the presumptive nominee. His press statement reminded people Clinton stole the election or cheated against Sanders. Reminded people why they hate Clinton and allowed publicly be against the debate. So Trump steps away from the debate after damaging Clinton and no loss to Trump. If Trump was a general he would be Alexander the Great - because he is unconventional and brilliant out of the box when it comes to media politics.