You are not getting what Trump is doing - just talking about the debate is what Trump wanted - it diminished Clinton without him having to debate Sanders at all.
“Truth is great and will prevail if left to herself, that she is the proper and sufficient antagonist to error, and has nothing to fear from the conflict, unless by human interposition disarmed of her natural weapons, free argument and debate; errors ceasing to be dangerous when it is permitted freely to contradict them.”--Thomas Jefferson
It seems Jefferson expected folks would CONTRADICT the errors made by the nature of the uniparty instead of exploiting (or "diminishing") them.
Exactly my take.
Trump just 1) proved Hillary a coward, 2) helped Bernie in CA, and 3) got a blast of PR!
Without doing much of anything, other than challenge Bernie to a debate. BRILLIANT.........