2016 will be very hard for the dems to win, and they won't win it by running last primaries loser.
She's not just last primary's loser, she's the most popular (active) political figure in the United States. She had as much support among Dems last go-around as Obama did--actually, more if you go by raw vote totals in the 2008 primaries.
More importantly, Obama doesn't have much of a clear heir apparent. Biden isn't going to get traction in the primaries. Clinton is probably the most visible face in this administration after O himself, so it would fall to her. That means if Obama leaves on a reasonably high note, she's the one who can cash in on that. She ran on experience and insider credentials last time but lost to the outsider promising change. It's unlikely someone is going to emerge in the Democratic party to run an Obama-esque campaign in 2016--it's very odd to run a change campaign at the end of two terms of your
own party controlling the White House.
Meaning if the Dems keep the White House in 2016 it's very likely to be because people more or less approve of Obama's stewardship--exactly the situation that gives a leg up to the experienced heir apparent. If Dems have to start the 2016 campaign by distancing themselves from Obama, they're starting at a disadvantage and probably would lose anyway. That wouldn't be a Hillary year and it wouldn't probably be any Democrat's year. If they're going to win, my guess is that it's under conditions that would be extremely favorable to Hillary.