There's a strong case for building a Left alternative to the Democrats...but history has taught us that third-party presidential campaigns can never do that.
Instead, Greens and other small Left parties should be focusing on doing what the Left does best- building from below, by heavily contesting local and state races where the contests are decided by popular vote, and working for electoral reform measures like ranked-choice voting, instant runoff voting, or proportional representation.
It makes no sense that, since the 1990s, the Greens, in particular, have refused to center these ideas and instead focused on the pointless objective of defeating the Democratic presidential ticket for the SAKE of defeating the Democratic presidential ticket.
It's been almost 30 years of wasted effort.
Why would anyone argue for staying with that approach?
If a tactic fails, it fails- at that point, an effective Left would abandon the tactic and try something else.