Something else to keep in mind is that presidential elections tend to tighten down the stretch run. So an 8-point Biden lead today would translate into around a 6-point popular vote victory on Election Day, on average.
Of course, you can use a prior other than zero based on the economy or incumbency or what not if you want. Given recent improvement in economic data + high polarization, the prior our model will use is currently something like Biden +2, which is not so different than zero.
But the empirically-driven intuitions that (1) elections tend to be close in a highly polarized era and (2) Trump will tend to win most of the close elections because of the Electoral College, are things that should keep people from being too overconfident about Biden's chances.
