Edit: Although really, what stronger evidence will you ever get? I suppose you could get a personal account from someone you trust (but that's not something you're going to hear if you're in the habit of brushing off such stories - most commonly, "honey, are you sure you're not just misinterpreting him?"). Barring that, a constant stream of similar rumours is the only evidence of a problem like this you will ever hear - until there is a "Susan Fowler event". Update accordingly.
* One needs real and credible evidence to carry through with depriving another person of any liberty or property.
* Probable cause never trumps or adds to evidence -- if you investigate and find nothing, that's it.
One of the most confusing and unfortunate memes of the past year is this "believe women" stuff, because it confuses strong suspicion with proof. If these cases show us anything, it's that there is plenty of real and credible evidence -- no need to go on intuition or suspicion.
...the idea that it isn't evidence because you don't
like it is not actually how the legal system (or
culture) works.
It certainly depends on your position relative to the person; but many people who are in a position to do something are also in a position to investigate -- they can take correspondingly stronger action, on the basis of an investigation.
For example, Teo had plenty of resources available to him, if he wanted to follow up on the rumors he had heard. He just chose not to do so.
The same is true at Uber: within the company there was a wealth of information available, and it was certainly feasible for Kalanick and many other people to look into the available complaints, emails, performance reviews and transcripts to determine if something was amiss.
It is true that people outside of Uber, and people not as well connected as Teo, would not have these resources available -- but then again, those people are not actually going to go into Uber and start firing people (for example), though they might refuse to associate with people they had heard rumors about.
When you say "we should treat 'a constant murmur' as much stronger evidence than we currently do.", I would ask you, evidence with regards to what? What would you do based on this kind of evidence?