A Greens/DSA/BernieBro alliance could seriously hold the election hostage.

A hostage situation is a negotiation. There's no hostage situation here because there's no chance Biden will give the left anything it wants. He can't actually do anything until he's elected, and if he's elected he has no obligation to keep any promises he makes (and obviously won't). The primary was the negotiation, Bernie was the compromise, and we lost. All we can do is (maybe) spike the election at this point, and "if I don't win (and there's no chance of me winning), you don't, either" is not a tactic that sounds reasonable to anyone outside of your group. A negotiation sounds reasonable, but again, we're past the point of negotiation.

There are four outcomes here:

1. Biden loses with the left voting for someone else. The left will get blamed, there's a small chance the left will be able to effectively communicate their reasoning, and everyone else in the Democratic party will reject that reasoning; see above. Even worse, the libs will trot out the "oh you could have negotiated to push Biden left" bullshit, knowing full well that the negotiation ship sailed sometime around Super Tuesday. The "leftists will never come out and vote for Democrats so why even bother" narrative will be strengthened.
2. Biden loses with the left voting for Biden. The left will still get blamed, of course, but it's possible that left-bashing won't catch on because it's too far from reality for libs to accept. After all, they tried to make left-bashing the scapegoat for 2016 and largely failed. Why? Every time it was brought up it was followed by "Bernie did 40 campaign events for Hillary and more Bernie voters came out for Hillary in 2016 than Hillary voters came out for Obama in 2008." Libs will swallow a lot of shit, but stuff like that is too indisputably true for them to deny.
3. Biden wins with the left voting for someone else. This is potentially the worst-case scenario, as, to libs, they've proven they don't need the left. Not only will the left not show up to vote for Democrats (a constant left-bashing talking point), but Democrats won without them anyway. Democrats will view themselves as fine with their new neocon friends who totally won't slit their throats at the first opportunity. Biden either becomes Bush III or an even less-effective second-term Obama. Three Supreme Court justices die and Biden either nominates clones of Antonin Scalia or doesn't get to pick anyone at all.
4. Biden wins with the left voting for Biden. Biden has the same presidency as Option 3, but the left is (to a degree) inoculated against getting bashed over at least this election (similar to Option 2.)

There are no good outcomes. Not voting for Biden certainly won't produce anything good; don't fool yourself. You don't get to negotiate when you've already lost, and even if you can pull off an electoral murder-suicide that's not going to do anything productive.

Voting isn't some moral imperative; it's a (limited) political tactic. Voting for Biden is bad (again: there are no good outcomes on the table), but win or lose it likely gives the left the best chances in any future elections. For spiking the election to work we'd need to effectively communicate our reasons for doing so (we can't), the libs would need to step away from reflexive left-bashing and have a genuine moment of introspection (they won't), the libs would need to decide -- for the first time in decades, at least -- to play nice with the left in the immediate aftermath of the left putting a stake in Biden (they won't), and we'll have to make the case that next time we'll come out to vote for a Dem (despite the fact that we just did the opposite of that, and only if we like the candidate).