Even if he were to step aside, whoever replaces him would be facing an uphill battle. Is it even practical at this stage?
It's possible, but not ideal. I might be wrong, but I think there are even some limitations to what funds would be available to the new candidate - I'm not sure they can use whatever Biden's campaign has raised so far. And then there is the question of building national name recognition in four months, which isn't easy.
The logic for it working would be that the story around who gets selected would create a lot of organic media coverage and possibly excitement. But I'm not sure I really buy into that. High attention voters would maybe follow along, but the average voter wouldn't. And it opens up for a lot of possible attacks from the Republicans. I bet they would go with messaging like "They selected this person out of desperation, they are not ready, etc.".
I think no-one really knows what the smart decision is now to maximize the chances. No polling or focus groups can give a definitive answer to that. The Democrats are in a bind no matter what. But I'd say that if they stay with Biden they are very likely to lose, and then they might think "We should have tried someone else". Conversely, if they try someone else and lose, would anyone really think "We should have stuck with Biden?". I don't think so.
Personally I think there might be a small messaging benefit to switching, in that the population might take kindly to something they consider common sense. A lot of people think "Are these really our two options?", so if the Democrats presents something fresh, it could play well with some.