Innocent until proven guilty is a great standard for criminal trials but probably not for the workplace, where people who report harassment are usually taking an incredible career risk by doing so.
Here's a relevant scenario: I'm a man, I hold a managerial position and I have to choose between hiring another man or a woman as a one of my direct-reports. Let's say both contenders have the same qualifications, they're both equally fit for the job, but then, I, as a manager, start thinking that if I hire the woman she will then possibly think of filing a sexual harassment complaint down the line in order to take my job. Remember the "believe" part, which means that the sexual complaint doesn't even need to be backed by anything real, because most of the times after the complaint has been made public the damage is already done for me, as a male manager, no matter what the investigation finds (if it manages to find anything). So I choose the man over the woman as my direct-report.
The more general point would be, be aware of the second order effects of changes you promote. For example, revocation of innocent-until-proven-guilty would almost certainly reduce the chances of people who have blown the whistle of being hired again, as they would present a much higher risk to prospective companies, regardless of whether their claims were true. The fact that they would blow the whistle at all would serve as a huge red flag.
Of course, I don't actually know the incidence of sexual harassment at workplaces, and have no idea whether false reports are even an issue. I strongly doubt they are, but I can somewhat understand male managers fear of them, given that they can ruin careers and marriages. As much as sexual harassment is a problem, I don't think assumption of guilt is an appropriate or long-term effective solution.
b. Because the costs of a false negative is forcing someone to continue working for/with someone they clearly feel uncomfortable around.
(compare to crimes, where it's much easier to protect the victim even without any sanctions against the accused)
Anyone accused of sexual harassment should have a chance to give their side of the story and an investigation should be performed, but the company should act based on what HR believes happened, not just what HR can prove happened. Neither the accuser nor the accused has the presumption of being right or the burden of proof.
Also note that it is in general impossible to prove that something did not happen, as opposed to proving it did, as things that did not happen have no evidence of their occurrence by definition...
This is incorrect. "Innocent until proven guilty" and "preponderance of evidence" are not two levels on one scale. They are distinct legal principles which can be applied at the same time or separately. Both, however, relate to the burden of proof and differ between criminal and civil trials, which is probably the source of the confusion.
Crudely: one standard is about what happens without sufficient evidence, the other is about how much evidence is sufficient.
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The burden of proof is a general concept of which party in a trial must prove their claims, and to what standard. "Presumption of innocence" is one possible answer to "which party", while "preponderance of evidence" is a possible answer to "what standard".
The party bearing the burden of proof for an issue is the party which must provide evidence. Simply: if everyone at the trial rests without offering any evidence, whoever bears the burden of proof loses. In criminal trials, this is the presumption of innocence you mention. In civil trials, the matter is more complicated, but in effect the plaintiff bears the (initial) burden. In asset forfeiture cases, infamously, the government bears the burden in the initial trial (against the asset), but the owner bears the burden of proof as a third-party claimant if they want their property back.
(It's emphatically not true that neither side has the burden of proof in civil trials. There is always a burden of proof when a claim is being made, to determine what happens if no evidence is provided. However, civil cases frequently involve affirmative defenses, in which each party bears the burden of proof for the claims made by that party. Both, not neither.)
The standard of proof, meanwhile, is the hurdle which must be cleared by whoever bears the burden of proof. It's how convincing their claim must be to be accepted. In civil cases, yes, this is a "preponderance of evidence", interpreted as the claim being more likely than not. In criminal cases, this is "beyond a reasonable doubt" - that's on the same scale as preponderance. Other standards exist outside of trial settings: in various contexts US law employs standards like "some evidence" and "reasonable suspicion".
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I'm not (just) being pedantic here. If we're talking about borrowing a legal standard for deciding against someone in a dispute, I think it's very important that we're clear on what we mean.
It's possible to vary both the size and placement of the burden. We could believe the accuser (burden on the accused), but hold the accused to a mere "some credible evidence" standard for their defense. We could believe the accused and demand evidence "beyond a reasonable doubt", or lower that to "preponderance of evidence", or even further. Or we could even use some standard not borrowed from the courts.
But right now, almost no one clarifies what they mean. The results of moving the burden will be very different from the results of lessening the burden, and it's important to understand what we're proposing.
Innocent until proven guilty is the standard for depriving someone of their freedom, not their job. If the harassment rises to the level of sexual assault, it's the standard the prosecutor will use. Otherwise, it's irrelevant in the context of sexual harassment.