The people who aren't guarunteed to vote democrat not matter what fall into two major categories 1) anti-genocide center left. These people want to be able to vote democrat, but simply can't compromise so far to the right as Biden has gone. Pressing the brakes on Israel support is not a super high bar, this group is looking for any sign that the Democrats are capable of listening 2) people who saw the debate performance and think Biden is too old.

Neither of these groups are hard to capture, and the reason most people aren't strongly calling for any specific candidate is probably because everyone understands it would foster divisiveness and be pointless. The most logical choice would be Harris; she would get access to Biden campaign funds, she's been in the whitehouse, she's a woman when abortion is on the ballot, she's responsible for Biden's support by black voters and she has distanced herself from Biden's braindead support of Israel. And, of course, she's not ancient.

So if I was speculating I would say it would be her who would quickly move to the front of whatever convention process might happen if Biden stepped down, but more importantly I think people are being unspecific because they don't need a specific person, they need "Not Biden" because at this point Biden is a surrogate for a Trump win. So think of it as "Not Trump".