Off the top, at this point will not be voting for any of the "big 3" come November. My results on political matching quizzes look like a WoW addict's freshman transcript. However, still quite interested in the process if nothing more than a spectator:
I think it's fair to say Donald is in stumble recovery mode following an on-air, emotional response. While he may emerge from this unscathed and even stronger, I think selling it as a pre-planned "4D CHESS" move is a bit too much credit.
That being said, I think his response (whether it's in 3rd grade English or not) was a smart tactical move:
He shows weakness to Bernie (and Bernie supporters). Is Trump scared of Bernie? I think in light of recent polls, and with Clinton's week, it's safe to say he's the more dangerous opponent to Trump. So generally scared? Maybe not. But more scared of Bernie than Clinton? Certainly. But Bernie needs a miracle to get the nomination, Trump avoiding the debate is a step to avoid that. BUT, Bernie & Co, who are now convinced they are Trump's kryptonite, will be emblodened to attack Clinton with a fury to the (seemingly insurmountable) end and/or, being SO convinced, mount a third-party campaign. Both are win-win, hands-off benefits to Trump. Only way he loses is if Bernie pulls miraculous upset, w/o Dems appointing a new candidate (Biden, etc). Pretty good odds.
Trump acknowledges the Dem system is rigged. This again, either prompts a 3rd-party run from Bernie & Co who are convinced they'd have better results outside of the rigged system, OR, those who are more pissed at the system than Trump, sympathize with Trump as the only outsider strong enough to beat the system. Again, Trump wins.
He propagates the idea that Hillary will 'probably' be the nominee, and that the 'rigged system' has already picked 1st and 2nd place. He's using this buzz to help nudge his weaker opponent forward to ensure she gets over the line by helping every see its between him and Clinton, and remind everyone that Bernie is a popular guy always doomed to fail in the DNC system.
To be sure, the flip-flop hurt him. And if it was obscure establishment RNC/DNC rules that were keeping him from debating, he'd be leaning on them hard ("I'd LOVE to, but the establishment has me handcuffed"). But this, too, will pass come the general, and seeing as Hillary also ducked the debate, I don't really see how it can be a huge sticking point. Ideal? No, but not a bad save.