If Ford had had a more detailed, highly plausible story backed up by a lot of circumstantial evidence, though still lacking the hard proof that might have put Kavanaugh at risk of actual criminal conviction, do you think that he would have been confirmed, or that he should have been?
'Proven' guilty was not required here because it was not a criminal case, merely a decision about whether Kavanaugh deserved the immensely important and high-profile job he had been nominated for. If you insist on using a legal phrase, I believe 'preponderance of the evidence' would've been enough for most senators in this case, though of course every senator is free to vote based on whatever standard they choose to hold the nominee to.
Moreover, with his attitude during the hearings, Kavanaugh did actually provide a good reason to reject him that had nothing to do with his guilt or innocence of the sexual assault - and it seems Heitkamp and Murkowski made their decisions primarily based on that, not because they necessarily thought him guilty, or more likely to be guilty than not.
I respect the decision of Collins, Flake and Manchin, and I don't think the Republicans should be demonized in such a way for these votes, but if I'd been a senator and free to vote my conscience without regard to party strategy, I do think that Kavanaugh's attitude in the hearings would've pushed me from a likely yes to a no.