I suspect that a lot of the people who have said, "I can't vote for Biden because of his support for policies A/B/C/etc" (and rhetoric and platforms are one thing, actual votes and bill sponsorships are another, because the former are words and the latter are actions) don't like that that's the stance to which their consciences have driven them. I would be very surprised if more than a tiny percentage spite-voted for Trump.
And the other thing that I think your analogy misses is that the Obama/Biden status quo
sucked for a lot of people. What swung 2016 to Trump? People who had
twice voted for Obama, and then switched to Trump because Obama hadn't made their lives any better--if anything they'd gotten worse, and they feared that their lives would get even worse than they already were if certain policies Obama advocated for (such as implementing the TPP, which Clinton was calling the "gold standard" of trade deals until Sanders pretty much forced her to be against it, and which Obama continued to advocate for even then) were to become law.
Trump isn't the disease. Trump is a symptom of a deeper rot that's been festering since the 1970s at least (
as noted by Noam Chomsky), and that rot has been sustained by and growing under the leadership of politicians such as Biden. It's the rot of a system that produces "socialism for the rich and rugged individualism for the poor" that's causing the damage. A continuation of that system, such as under a Biden presidency, would only serve to produce another candidate from the far right like Trump, except one who isn't as obviously grotesque, incompetent, and idiotic as Trump, and won't be so blatant in his attempt to implement fascist policies so quickly--someone like Viktor Orbán, whose first government (1998-2002) was generally considered a pretty standard European conservative government by my understanding.
The only equivalence being presented, essentially, is that the answer to the question, "Gun to your head, Biden or Trump?" is, for left-wing voters who vote on the basis of ideological agreement and conscience, "The bullet."
Biden might be better overall (though worse than Trump on a few select issues), but still sufficiently bad that a vote for them is unconscionable.
And when you tell someone to ignore their conscience--which is what telling them to vote for Biden is--don't be surprised when they tell you to fuck off, don't be surprised if they dismiss you as a partisan hack, and don't be surprised if it makes them less inclined to vote for any candidates from your preferred party in future.