But I think the best way to assure voters that Biden is fit for the job he's doing right now is to simply get him out in front of voters and show that he does have what it takes - whether that takes the form of rallies, TV interviews, and whatnot. Because valid or not, people have those concerns and those concerns need to be alleviated before the election. But the party itself playing along with notions of a replacement play into those fears and give them more credence than they might otherwise if the party itself would give its candidate a better showing of solidarity from the start.
I wouldn't say the party itself is "playing along." Yes, a handful of public officials have voiced concerns, but the party overwhelmingly has maintained its support behind Biden. They even pulled out Obama, so I don't think the party is showing signs of faltering.
I totally agree that rallies, TV interviews, and other widely public appearances are the only way (I can think of) to reassure voters. That hasn't happened yet though. I know he's doing a TV interview soon, which is pre-recorded. Not that pre-recording interviews like that isn't standard or is an admission of unfitness, but I also get how voters wouldn't feel reassured. Biden has largely spent the past week in private meetings with donors and officials trying to reassure
them. While it's far from clear, media reports of those meetings haven't even been positive. Biden can still turn that around, but I do think the clock is ticking on leaving a permanent impression on voters.
I agree with what you said, those things just haven't happened yet and won't reassure voters until they do. When and if they do, that's a better response to concerned voters. Clearly it's an issue now and acting like it's not an issue at all will only exacerbate its effect.