I think one of the biggest problems for the GOP if Jeb's the nominee is that 2016 will become much more of referendum on Clinton vs. Bush than on Obama. That's not a problem if by election day Obama has high approval ratings and the economy is in great shape (although this would automatically favor Hillary anyway), but it makes him one of the worst candidates to capitalize if things go sour. A fresh face could make the case there needs a change from Democratic policies - tying Hillary to Obama - if that happens, but it'd be much more difficult for Jeb to convincingly argue that going back to another Bush would be the solution.
Good analysis. Bush v Clinton means it becomes the 2000s v 1990s and the 2008-2016 period is ignored.