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.

Yeah those are basically the Scenarios with two big corrections in scenario #2:

• The next will probably be worse than Biden since the DNC, upon seeing that lefties will even vote for a candidate that supports a quasi-Nazi regime activelly commiting a Genocide will likely conclude that they will not rebel not matter what, so expect an even further shift to the right of the Democrat party.
• The Democratic Party is not centrist, not even close: it's pro-Oligarchy, which is an anti-Democratic hard-right position (anti-Democratic because it places Money above The State, which is the Power that voters supposedly control hence gives primacy to Money and those who have most of it, hard-right because defending that those who have most Money get the most Power and choices is in direct opposition to Equality, even just that of Opportunities).

As for the DNC not being able to stop a left of center candidate, just look at what they did against Sanders, even before counting the super-candidates which were going to vote against him anyway and override the popular vote. The idea that Obama is in any way, form or shape left of Obama is hilarious for anybody who, like me, was in Finance at the time of the 2008 Crash and had a front row seat to see how exactly Obama unconditionally saved the wealthiest people and made everybody else pay the price - just because the guy is a true political songbird who makes amazing speeches doesn't mean "the greatest good for the greatest number" - the core principle of the Left - is even in the tinyiest of ways part of his principles. The Clinton-vs-Obama primary was a fight between two kinds of neoliberals that put in opposition two factions within the American Elites, not a fight between somebody representing the average American and somebody representing the Elites.

We don't know really how bad a Trump presidency will be, though we know for sure just how far to the right are Biden's principles, but yeah, you are right that a Trump presidency might (it's all speculation until it actually happens) be incredibly destructive, which is why I pointed out in my comment (last paragraph) that it's definitelly a risk and people should consider all things in their voting decision.

Personally I think either of them will lead to the death for good of Democracy in America, though doing it via Biden will probably mean it will happen with more steps, but that's just my opinion based on the trend so far (and, that I expect that a guy who supports what are basically the modern Nazis whilst they mass murder civilians because of being from another etnicity is either a sociopath or an extreme racist and that means he will just as happilly fuck up the lives of Americans - though, no doubt, unlike Trump he will be telling them that's not what he's doing - just as as he is right now happilly helping out murdering en mass Palestinians: good people don't knowingly help others commit mass murder).

I might be wrong on all of this and even if I'm not, not being American or living in America I'm way more isolated and have no real stake on that choice, so I openly admit that I have the priviledge of being able to hold a Thinking Person's highly intellectual position on this because either way it impacts me very little personally, so I can just analyse the whole situation and point out the broader implications of the voting decision for a leftie and the profound hypocrisy of the Propaganda which tries to deceive people with the idea that it's a simple consequences free choice, with no real additional risk either way for my own future.