"a generic USERLAND 'libusb' equivalent for Windows"
Sounds a bit like WinUSB, which has existed for about a decade, but the story specifically for USB/RS232 devices is even more interesting. There's an official way to do RS232 over USB which non-Windows platforms have supported for ages and which Win10 now finally supports out of the box. So, if that's what you are using your FTDI chips for, then on Win10 you won't actually be using FTDI's drivers.
On older versions of Windows, of course, you are a bit stuck and nobbling the end-user's hardware when the counterfeiting offence was probably committed *way* up the manufacturing chain strikes me as very unfair on end-users. Fortunately, taking the law into your own hands like this is almost certainly contrary to computer mis-use laws and courts would in any case take a dim view of a vendor declaring themselves to be judge and jury like this.
Apologies for the long post, but the list of things that are wrong about this story is just boggling.