We absolutely should push for something better. There are lots of ways to do that.
Voting for Cornell West or Marianne Williamson for President is not one of those ways. They can't win, or make things better, but they can sure help make things worse.
If you want the Democratic party to be more progressive, great! I'm 100% on board with that.
The way to do that is to convince the median voter to be more progressive. Politicians will follow.
It's that simple, and that difficult. Protest votes don't move the general population.
And the effect of minor party candidates on the Democratic candidate is the exact opposite of what you want - their calculation, when faced with a minor candidate on the left is "ok, polling says that candidate is going to remove X% of what would be my likely voters in a 2-person race, so I need to pick up at least X/2% centrist Republican voters to make up for it. So how can I appeal better to centrist Republican voters?"
It's not that they won't try to win the votes back from your fringe candidate - but they won't try very hard, because adopting part or all of the fringe candidate's platform will turn off the non-progressive Democratic voters (and any crossover voters), and those are unfortunately a lot more numerous than the fringe party voters.
barfo