The researcher can only change the likelihood the data isn't obtained by a malicious actor if a malicious actor hasn't already obtained the data. The researcher usually has no way of telling if a malicious actor has the data. Optimizing for the worst-case scenario, which is yes, a black-hat hacker has already gotten there, it makes sense to prioritize notification of users by all available means so they can attempt to remediate the data loss.

Nearly all security vulnerabilities are published in blog posts at some point because most companies deny a problem even exists or needs to be fixed. Sometimes they just don't even respond and the person who discovered the vulnerability publishes anyways as a sort of "punishment" for the companies lack of response.