The same poll you are citing shows that Hillary's favorability rating is at its lowest point since June of 2008, and her unfavorability rating is at its highest point since June of 2008. (That's using CNN-sponsored polling's OWN past numbers - it's not comparing X to Y.)
So what? It's nearly the same as her favorables in May 2014. Regardless, even Hillary's numbers in June 08 are far superior to any of the Republican candidates.
In Obama's comfortable 2012 victory, he lost the white vote 59-39. So a 43-54 favorable rating among whites is pretty damn good. And you're overestimating how far minority turnout will fall. Blacks still turned out for Gore and Kerry after all. If Hillary wins 40% of the white vote, she wins the election.
Blacks did not turn out for Kerry, at least, nothing like they did for Obama. Bush got 11% of the black vote, McCain/Romney got 4 and 6% respectively. I expect the black vote to be more republican in 2016, simply because there will not be a black on the democratic ticket, and there are blacks, like it or not, who voted for Obama solely because of his race. I will be very surprised if the republican candidate in 2016 does not hit 8% of the black vote, and I can definitely see 10 or even 12%.
Following your logic, many women will turn out to vote Hillary because she will be the first woman President and they will more than offset any declining turnout among blacks.
Also, Clinton was much more popular among Hispanics during the 2008 primary and if she chooses a Hispanic running mate this will help her too.
