The base and the future of the Democratic party are minority voters, in particular, black women who vote every election and overwhelmingly for Dems. The problem with Bernie is that he could not excite this portion of the electorate, so he could not excite the true Dem base, which came out in huge numbers for Hillary.
Yes, Dems need to excite millennials, but we should do it through arguments toward equality, social justice and an economy that works for everybody. The fact is that HRC had plans for all those things, but the MSM would never let her make her statement because the BS emails story and BLOTUS sucked all the air out of the room. The media NEVER properly vetted Drumpf, they allowed him to run as his TV persona, so they never forced him to release his tax returns or a legitimate medical report.
Dems need to have an exciting younger candidate who is not afraid to get outraged over Drumpf's lies and call him on his BS, but can still make an articulate argument about why voters should vote FOR Dems. Dems need candidates like Stacey Abrams who can make the argument to a broad range of voters, including those who are truly disaffected.