What is the source of your confidence? Hillary has proven to be a dull, rather wooden campaigner at best, and she can't seem to shake off her detachment and very obvious sense of entitlement. That puts people off: the more she is seen the less she is liked. Meanwhile public trust in her continues to fall and her dull rhetoric on the supposed "war on women" has become very tiresome. Bernie Sanders can certainly excite the progressive wing of the party, but I think we both believe his ultimate political appeal will be too limited to win.
If Joe Biden enters the race he could add a little excitement and variety, but that would doom Hillary, and the resulting rancor could be very distracting. Moreover Biden will be an excellent target for all who have issues with the Obama presidency. I, could be wrong, but see more negatives there than positives for the Democrats.
There's very obvious democratic competition among the Republican candidates, and last night's debate suggests to me that the days of Trump's attention grabbing monologues may be ending. He isn't nearly so effective in a debate with alert opponents, and as the field narrows a bit, and as the leading candidates refine their games, I see rising appeal there.