All this celebration around Hillary being the "presumptive" nominee is premature. There's a story here and I'm surprised no news source has covered it. [update: I just found this Feb 24 article from Glenn Greenwald discussing exactly this point. Nothing but respect for him and The Intercept, and Pierre Omidyarfor funding this valuable journalism resource]. The problem is Hillary Clinton's popularity is on a long, slow steady decline starting 3 years ago. It seems the more exposure the public gets of Hillary, the more they dislike her.
If Clinton is the Democratic nominee, she has at best a coin flip's chance of beating Trump in November:
Now if I were a DNC superdelegate—even one who had already pledged for Clinton—I would have a long look at these numbers, and a hard conversation with my colleagues. See, this is EXACTLY the situation that superdelegates were created for: To sway the nomination toward the candidate who has the best possible chance of actually winning the presidency. To simply give it to Hillary—even if she has a slight edge over Sanders in the primaries—and then have her lose to Trump in November is a risk I wouldn't take. And if the DNC is approaching this rationally, they won't either.