1. If Obama replaced Biden, it would send two very clear signals: weakness and panic.
2. Biden has lower unfavorables than Obama. So it would make more sense to replace Obama than Biden.
3. No matter your politics, if you take an objective view, Biden has been a pretty good vice president. First, he’s been loyal, which is job No. 1. He plays the No. 2 role well, never seeming to overshadow the boss or scheme to take his job. In addition to being the administration’s liaison with the House and Senate, Biden has taken leadership roles in negotiating internally and externally on the Iraq drawdown, the debt ceiling, and the START nuclear-nonproliferation treaty. Whether the money has been spent to good effect or not, he also is charged with accounting for stimulus spending.
4. Biden is hugely popular with labor. Obama, not so much.