Yes, that's why it's a hard choice rather than a simple choice: there is a significant and genuine "now might be the worst time to do this" factor at play, though if you notice there is a "might" in there because that's still all in the realm of possibility and there are chains of consequence that might mean that the Trump-vs-Biden now will look like the "good old days" in the next election since it it's a valid scenario that after the lefties vote for a quasi-Nazis-supporter, the next candidate pushed by the DNC will be even worse and the candidate put forward by the Republicans after a Trump defeat is a competent version of Donald Trump - a full-on highly intelligent sociopath that uses the same tools as Trump rather than an incompetent Narcissist which at times is his own worst enemy - an even worse choice than Biden-vs-Trump.

Also the frequent repetition over the years by the Democrats of that same "now is not the time" argument, almost always followed by next time being even worse, makes people suspicious of all the assumptions put forward to support that argument by thos people, and that they're complete total bollocks just like the last 4 or 5 times those same people made that same argument.

Further, there are multiple paths to "Stop Trump" and the one where Biden shifts leftwards (especially by stopping unwavering support for quasi-Nazis mass murdering children) seems like a far simpler way to achieve that objective than expecting million of people to swallow their "though shall not kill chidren or support those who do it" principles to vote for a guy who keeps on supporting the mass murder of children.

This is not perfectly that Game Theory scenario: the approportioning of representativeness can be changed by the candidate himself after the candidate selection is done, so Fear of losing the election might be enough to achieve some leftwards shift and still guarante that both the DNC and lefties end up winners. In fact, IMHO, that would be the outcome that maximizes the upside for both as a group and possibly the idea scenario give the few real choices than can still be made: the DNC gets his man elected even if he acts a little bit more leftie and the lefties get a little bit more representation.

Everybody going "You have to vote Biden to stop Trump" is making that ideal scenario less likely because they're decreasing Biden's (and the DNC's, who can pressure Biden) Fear of defeat, whilst it's the people saying "I won't vote Biden until he starts supporting the unnacceptable" that are making the ideal scenario more likelly.