It does matter if Biden decides to stay in. It makes it far less likely the DNC is able to replace him, because the delegates are already pledged to him, and as far as I understand, in order to replace him they'd need a clear alternative singular candidate to nominate. Plus, they'd need to stop Biden from reviewing and selecting a loyal slate of delegates from each state who will certainly vote for him. It'd be much much more likely if Biden steps down and himself calls for a 5 or 6 week primary. My source, if you want to check it out. There's a chance I'm misinterpreting but I think that's the jist of it.
As for those people rigorously defending Biden, I don't necessarily think they're automatically pro-Israel. They're scared, for sure, as we all are. And they've probably bought some of, if not all, the Democratic posturing that Biden is just fine, and other writers' sentiments that doing something like changing the nominee last-minute will be a disaster because it is unprecedented, and so they double down. Like a superfan in sports who is in complete denial that their team has a glaring weakness and probably won't make it out of the playoffs. You see it all the time. I won't ever hold it against those people, because it's easy to be in that state, especially if you love your team. Plus, if you logic it out with them, sometimes they come around and end up seeing the roster change was actually needed. I don't think deriding them gives them a chance to see your side.
I think it's likely Harris would be the nominee if Biden stepped aside, unless they are spooked by her polls, as well. But that is a lot of ifs, and really doesn't bear much weight on the likelihood as I see it (and hate it) of Biden staying in the race and losing to Trump.
But yeah, that's why it mostly hinges on Biden, because the DNC has rules in place to protect the presumptive nominee from a delegate mutiny.