I'm guessing the most serious impact will be non-browser certificate-based authentication, such as RDP or line-of-business apps that use standard Windows APIs.

I'm guessing the most serious impact will be non-browser certificate-based authentication, such as RDP or line-of-business apps that use standard Windows APIs. Local files (ex. fake codesigning) can be detected using anti-virus and third-party browsers are not quite common in the workplace and for home users.