Replacing Joe would be admitting that Teh Awesome made a mistake appointing him in the first place. And Teh Awesome never makes mistakes. Replacing him now would be admitting that Teh Awesome thinks he could lose this election. And Teh Awesome never loses.
And Liz Warren would be trading one kind of trouble for another. And Hillary is at least as likely to bail out for 2016 as she is to switch from State to VP nominee in a campaign she could lose with a running mate she thinks she should have been president instead of in the first place. Now that I think of it, more so. No, Barry and Joe are stuck with each other.