It's not an issue of magnanimity, avarice, mercilessness, munificence, or any other large multi-syllabic word you care to use. It's an issue of honesty and integrity, two simple concepts that Hillary Clinton is severely lacking in perception of these days. She promised she would have a debate with Bernie in California. This promise wasn't contingent on how much she was winning or losing by, the phase of the moon, or how many banks there were on Wall Street that day. It was a promise without pre-conditions, and she has now broken it.
That promise being broken, Bernie is ready for a debate in California but has no one to debate with. Now Trump shows up and says, sure I'll debate you. Bernie wants a debate. Hillary won't play. So what's he going to do? The obvious thing is to have a debate, and Trump is available, so they'll debate. There's no deeper meaning here other than Bernie was owed a debate which wasn't pre-conditioned on whather he was winning or losing in the primary race and then his debate partner changed her mind. He still wants a debate and now he's going to get one with a different candidate.