It appears that Windows 9 will NOT have a Microsoft anti-virus program, but it will have some AV program. MS announced today that the final version of Windows 8 will include whatever AV their partners want to install. If the AV expires, Windows 8 will keep annoying you with reminders that you need to install Windows Defender. And you will have the option of deleting any AV program you want and installing another.
Why? It costs MS money in technical support when irate customers complain about problems that are caused by viruses, and it costs them money to keep Defender up-to-date. MS will be happy to abandon Windows Defender IF everyone has a good AV program.