Sadly this isn't a new problem for Windows8. Every version of Windows has had problems when old devices don't have compatible device drivers.
I don't think this is true of serial port access. AFAIK, you could talk to a serial port in every version of DOS and Windows with no problem up until Windows 8. Now suddenly, with Modern apps, it doesn't work at all.
Microsoft should have created an app capability for serial port access. Microsoft allows my app to become a network server, to impersonate me using my network credentials, and to access random removable storage. I would say all of those could be more insecure than accessing a serial port, which hooks up to hardware I have physical access to.