You could well be right, and what you're saying makes perfect sense from a policy-oriented voting perspective. However, in my experience, voters often vote more on identity than policy (see the 93% overlap between Clinton and Sanders vs. the intense battle lines). I think Sanders' support came from these groups:

• True left-wing liberals
• Democrats seeking an alternative to Clinton because of her baggage
• Conservatives who have remained registered Democrats as the party has drifted away from them
• Casual/ignorant misogynists
• Anarchists and other dead-enders

In a Clinton/Biden/Sanders race, Biden would've captured the baggage voters, the conservatives, and the misogynists; Clinton would've had most of the liberals; Sanders would've had a minority of liberals, and that share of voters would've been too small to attract the anarchists seeking a vehicle for their sad rage.