I think that he might have had a great chance. Sarah Palin was a complete outsider to most Americans. She was picked because the GOP thought they needed a gimmick to compete with a black Democratic nominee. Unfortunately for them, they picked someone with moral and ethical issues who also happened to be dumb as a brick. She cost McCain the election.
Aside from her dragging him down because of her multiple, calamitous mistakes, she made him and his camp look like they couldn't be bothered to vet a candidate for vice president. It's not possible that they had all the information on this woman and still chose her, no one is that stupid. Therefore, they must not have bothered to do their research. That is not a group of people that the public wants running the country.
There was almost nothing that John McCain could have done to win that presidential election. Barack Obama was simply too strong of a candidate for him to do anything at all. Add in the fact that there had just been eight years of a very "bad" Republican president and there was no way McCain could have won.
I do not think John McCain would have won the 2008 presidential election if he had not chosen Sarah Palin. McCain's problem was that he diverted from many of his progressive and centrist views and chose the hard right. This was a mistake after the two miserable Bush administrations. Bad choices.