Bernie Sanders probably thinks he can get more concessions at the convention (if we have one) if he remains in the race as he did in 2016, plus he probably doesn't want to disappoint his base. They would turn on him like a pack of wild rabid dogs if dropped out one second before he has to drop out.
I don't think it matters one way or another, I think the democrats have already made up their minds. How it all turns out with Trump, who knows? I sure don't. Never would have thought Hillary would have lost, but she did. However, since Biden did get a good amount of votes in places like Michigan and Missouri and Biden is not hated like Hillary was, I am somewhat more hopeful than I was.
Think of government as having the job of representing as many different points of view as possible in a way that doesn't erupt into war.
To facilitate constructive discourse, or at least to attempt to, they have to move different views around parties and different parties around different offices. Sometimes the GOP is the opposition and sometimes the Democrats are. Taking turns as the opposition causes them to develop their ideologies and prerogatives in a different way than if they were in the White House dealing with the other party in the opposition.
For every asymmetry that happens in government, there is going to be a corresponding asymmetry that favors the other party. So, for example, B Clinton had a Democrat congress to work with his first term, and the Dems took advantage of it to pass a lot of legislation. Then, the GOP took over the congress during his second term and eventually produced the Contract with America, got GW Bush elected, and then that party had a strong position and less opposition.
So really what needs to happen is that some form of balanced governmental formation needs to emerge/evolve where the two parties work together and honor their differences instead of always just trying to subvert them. It's like there needs to be a ceasefire in a war where one party or the other doesn't take advantage of the ceasefire to develop underground tactics to subvert the other party covertly while pretending on the surface to be getting along.
Real democracy is about respecting each others' views and differences as an inevitability of freedom of religion/speech/culture. But since there are differences/conflicts that can't be respected and honored for whatever reason, we have to keep struggling for common ground where doing so is practically guaranteed to be useless/fruitless.
Probably Trump will stay in the White House for another four years, though; and if he doesn't and the house remains leftist, there will be a lot of conflict with the senate and supreme court, and a lot of bad blood will result from putting the GOP through the misery of taking all those executive orders to court to get the supreme court to revoke them. Whatever is achieved by executive order will be subject to later retribution, as is happening now vis-a-vis all the executive-order work that was done by the Obama administration.
So really what has to happen is people just have to bite the bullet and see what they can achieve by finding common ground with the other party and listening to dissent that emerges from outside the two major parties.
Bernie's supporters are right for maintaining their stance, because voicing their opinions is more important than shutting up and rallying behind a Democrat nominee that doesn't really represent them.