This is what will happen if there is no credible voice (and by "credible voice" I mean an articulate candidate with legitimate credentials, but not necessarily someone who the odds-makers or pundits will say could prevail) to challenge Hillary from the left.
1. Hillary will move closer to Third Way thinking, i.e., staking out positions her campaign considers "centrist" and more likely to garner support from those who identify as Republican or independent.
2. Hillary will run a cautious campaign, trying not to offend anyone but inspiring no one.
3. Hillary will affirmatively anger the most passionate portion of the Democratic base, suppressing voter turnout (and hurting other Democratic candidates at the state and local level).
4. Hillary will win the nomination, but be ill-prepared for the general campaign.
5. The GOP candidate may well out-flank Hillary on populist themes, which probably won't result in a GOP victory but could muddy the waters as to which party represents The People best.
6. Hillary will probably win the general election, but be inclined to fill her cabinet with neo-conservative types and take more "centrist positions."
This is what happens with a credible, progressive voice in the primaries.
1. Hillary will feel pressure to move left on an assortment of issues during the primaries.
2. Hillary will still win the primary.
3. Hillary will be better prepared for the general election (and especially better situated to position herself as more populist).
4. Hillary will motivate the base, increase turnout, win by a bigger margin and help down-ticket candidates.
5. Hillary will appoint more progressive candidates and her administration will be less interventionist in foreign policy, and take more progressive positions domestically.