Now that we know that Manchin could have been beaten
No we don't. We know that the polls showed him in serious trouble at one point, but we don't know if the polls actually reflected reality. The final result wasn't even close, which should give pause for thought.
Of course back then he wasn't a Senator with a record to run against.
All I can say is that Raese, a horrible candidate whose campaign made a number of terrible missteps, lost by 9 points. It's certainly not ridiculous to think that a real campaign mounted by a credible, well-liked Republican could have made the difference.
If you seriously think that Raese was up by a decent margin at one point (and I do), you have to further believe that Capito would have posted those kind of numbers or better at the same time. And if Capito was posting those kind of numbers or better, it'd have given Democrats pause as to whether or not to spend the money they did on Manchin.
Of course, it's possible that lightning could have struck twice -- that Capito's campaign could have been derailed by a Craigslist ad looking for hicks. By the same token, that kind of attack wouldn't have worked as well on her, since she wasn't the same kind of out-of-state phony Raese was.