What if Bush II decided to stay governor of Texas instead of running for President? Who would run in the primaries? Who would get nominated? Who'd win the general election? How would Bush II's governorship be seen?
I disagree that McCain would inevitably be the nominee. The only reason we didn't have more people running that year was because Bush shock and awed with his fundraising. It scared everyone off. Without him in the race, it opens up the field for other candidates like John Engler, Lamar Alexander, maybe Jack Kemp, and who knows who else. It opens up a whole can of worms.