McCain was not a weak nominee. He was stronger in 2000 than 2008, but his real downfall was Palin.
On a strict + / - scale, she brought in more voters than she cost him. That's undeniable. McCain had zero gravitas with the conservative wing of the party (including fiscal conservatives who would eventually become the Tea Party). Palin (rightly or wrongly) energized the conservative crowd. There was also a very strong anti-Obama crowd, but no one was saying anything bad about Obama during that election. Not in the press, who was selling him as a once in a generation messiah who would solve all our problems, and definitely not from Pops McCain (who was a bigger defender of Obama than even Hillary was at the time). There was a groundswell of people that knew Obama would be an unparalleled disaster and Palin was the only one that was grabbing the megaphone and saying it. Now it was unfortunate for McCain that he thought he could control her (or otherwise "train" her), which would hide her paper thin grasp of the issues. If he muzzled her, she couldn't bring to the table what she was added to the ticket for (energize the mud-flap, six-pack, flag-waving, fun-loving vote) but if he let her run unleashed he'd be doing more damage control than actual campaigning. It really seemed like McCain's team simply vetted her resume. On paper, she's fantastic. Mayor. Governor. Former point guard. Working mother. Shoots animals. Good looking. But any kind of intense interviewing would have exposed her, so the "team" didn't do John any favors. Once it was done, it was done. But given what she was, what she was brought on the ticket for, and what they knew about her, that team needed to keep her locked in on the right. Do all the Fox stuff that McCain is terrible with. Have Palin only talk to the Hannity/Rush/Savage type shows and keep her away from mainstream outlets that were going to go out of their way to edit her to make her look like a dunce. Let McCain sell the ticket to Katie Couric and Matt Lauer, the crowd he was more comfortable with. Instead, they did just the opposite. Cue McCain in a bathrobe at the end of Ferris Bueller's day off. Go home. It's over.
With all that said,
nobody was beating Obama that year. With a complicit media selling his candidacy, an energized black vote, and an enthusiastic youth base wanting to elect the first black president, he was a lock.
That was such a wasted election cycle for all parties involved. Hillary would have made a much better President at that time, as she would have drifted toward the middle and worked on jobs/unemployment and stabilizing the economy first and foremost. Any kind of central hand at the rudder would have brought us normally out of the recession (as we had every recession before that). Palin wasted a golden opportunity to become well-versed in the issues and would have essentially been doing what Trump is doing now (and probably would have been the nominee this year), but she is SOOOOOO incredibly non-curious to the point of exhaustion. While I don't think she's smart, she's not dumb either. She's simply content to be what she is, and that's not good enough to be the President. Obama had the opportunity to be the Reagan for the left, but he sold his legacy out for the Affordable Care Act. Well, that and he's a petty little ##### scumbag narcissist.