I updated my little doomsday counter (sidebar right) today. It now knows about Windows 10.
I also reprogrammed it so it will run automatically for the next eight years, which is when Windows 10 extended support expires. Mainstream support for Windows 10 ends in October, 2020, a little over three years from now. Presumably, we'll know about Windows 10's successor by then.
Or perhaps we'll discover that Windows 10 has become software as a service and will continue forever.