I'm not an American, but I am very much a Sanders supporter (and would very happily vote for Warren if she became the candidate). Biden is way down on my list of people I would like to support. Here's my attempt to sell you on Joe Biden anyways.
Let's assume that the bare minimum President Biden gives the United States is a rollback of the most egregious Republican policies of the last few years: no more kids in cages, lifting the immigration bans, maybe beginning to restore the bureaucracy that got hollowed out (like the State Department). Pretend he does absolutely nothing else during that time, enacts no policies of his own, doesn't try to push a single thing beyond what already exists. Short of a major accident, health issue or assassination, Joe Biden will be President for the next four years, and even if he does die in that time, the Presidency will be in the hands of a Democrat.
In this scenario, Joe Biden is still giving you the gift of time.
Two years of Obama-era immigration policy, while certainly not perfect and definitely not a deportation-free policy, means two years to prepare for the possibility of it happening again, to build organizations and communities better able to protect against its people being stolen by ICE and imprisoned for no good reason. Two years of building up the country's civil service again can mean two years of reinforcing something we now know can be lost so easily due to neglect or passive malignancy. Two years of protection for the Affordable Care Act means two more years that Americans will have slightly less shitty healthcare than they could've had under Trump. That's thousands of people that get to be alive. Two years of a Biden administration directing Supreme Court appointees... well, maybe they'll all be Merrick Garland'd, but at least we know what that looks like, and at minimum we won't be filling those lifetime appointments with right-leaning justices in the meantime.
Even if you assume that after two years Biden's administration will be so weak that Democrats lose the House and Senate and then the Presidency in 2024, Biden will have given you time that you do not currently have. Four more years of Trump guarantees that he will move forward on his agenda, emboldened by a renewed mandate and either unshackled because he no longer has to worry about re-election due to term limits, or unshackled because he no longer has to worry about term limits. And yes, this is partially a "he's not Trump" argument. But let's be clear about what that means. It's not "well we got essentially the same guy except he doesn't spray tan himself." There is a meaningful difference. More importantly, it means the activist communities that have formed around people like Bernie Sanders and Elizabeth Warren can continue gathering resources and planning for the future. Building coalitions, pushing downballot races further left, keeping Medicare For All and corporate malfeasance in the spotlight--all these things are easier to do when you're not simultaneously busy trying to fight the existential threat that is the current administration.
The biggest mistake we made as a society in 2008 is that we thought the job was done once Obama won. We were wrong. Some people, who have suffered longer and harder than most of us, knew this implicitly and tried to make us see the error of our ways. The job is never done, victory is never complete. Even if it accomplishes absolutely nothing else of value, a Biden administration buys you time, and that can be the most valuable commodity of all.