How can we miss you if you don't go away?

This is actually good for the country.  His attendance would be a giant distraction.  He's going to do something else during the debate that will draw media attention, but it won't actually matter.  His base wouldn't be tempted by any other candidate no matter what.  People who are curious and possibly disposed to voting for someone else will watch the debate and be educated about the other candidates without him sucking up precious time and starting pointless arguments.

He'll figure that out after the debate when news coverage and talk is about the people who were there, so he'll be at the next one.  But for this one, this is the best possible situation.