There are ramifications to just dropping a VP candidate. The delegates have officially nominated both of them, by name, so it isn't just a matter of Obama shoving Biden over for Hillary. I am not sure of the exact procedure (it is pretty unprecedented), but it would -- at the very least -- involve the DNC changing some rule, some how. That, in and of itself, would be a spectacle.
Things that would humiliate and ruin a conservative Republican don't even cross a Dem's radar screen. So — gosh, we could just let our imagination roam. Biden could say he knew he wasn't helping the campaign, the issues are too important, McCain too dangerous, he loves America too much, yadda yadda yadda — and so, for the good of This Great Country, and the wonderful Obamessiah, he will fall on his sword to make way for a much better and visionary candidate, blah blah blah.
We'll never be prepared if we think the Dems will be fair or rational.
If the last sixteen years have taught me anything, they've taught me this: power means more to Dems than anything in the universe.
The rules are the same as when Eagleton dropped out in 1972. The DNC picked a replacement. But the Eagleton situation and replacement was a disaster, and the Democrats know it. Biden is in it till the end, and nobody is even thinking of replacing him. Both candidates have their running mates and both are stuck with them.
In the old days, politicians would work the convention delegates to be selected as the VP nominee.
This idea of there being a partnership between the presidential candidate and the VP candidate is a relatively recent phenomenon. It came about because the primaries now nominate the presidential candidate, allowing his delegates to dominate the convention. So unless the VP had the endorsement of the presidential candidate, he wouldn't get picked.
But the fact remains that the VP nominee is running for his own office, and that means that Joe Biden is the only guy who can really decide if he is to step aside.