I say this every time someone suggest just that, but...so long as Obama is black, there is NO WAY he can be unseated as the Dem nomination. As much as his approvals have fallen, he still maintains something north of 90% black support, and 75% Latino support. Because of the demographics of the Democrat party, it's virtually impossible - mathematically speaking - for Obama to lose the nomination with that kind of minority support.
I suspect Hilary understands this dynamic, which is why she won't run. BUT, I could see Biden becoming "sick" and being replaced by Hilary. That's a real possibility, if she wants it.
I like your analysis. The Democrats have hitched themselves to an electorate whose stupidity they have carefully cultivated for generations by trapping them on the Welfare/Government School Plantation. Now that electorate is so racist and stupid that the Democrats can’t free themselves from even a moron-Communist like Obama.