Unless you think this would actually lead to banks taking such vulnerabilities more seriously in general--which I don't believe is the case--taking an action like that is pure spite. Consider the possible outcomes for this particular vulnerability: [1] nothing happens, [2] it gets heavily exploited, customers lose money, and it doesn't get fixed, [3] the same thing happens and it does get fixed. In all three cases, the outcome is at least as bad as it would have been had you done nothing, except possibly earlier and worse.

I really take issue with the notion that security is important, so you're fully justified in screwing people and companies over as much as possible to prove a point. That seems to be a common attitude in the security community. I get the frustration people have with the intransigence of corporations and programmers, and people's general stubborn unwillingness to understand the severe impact of vulnerabilities, but if just security-shaming companies into fixing bugs actually worked we would have a much more secure internet today than we actually do. Unless you can get regulatory agencies to start holding companies and individuals legally accountable for security issues (that is, making it more expensive not to fix than to fix), nothing will change, even if you have all the technical solutions and social pressure in the world.