I plan on keeping Win 7 on my dual boot system as its needed to transfer patterns to my wife's Singer Embroidery machine. I have a USB/RS232C adapter but it will only work on Win 7, not Win 8 and not on the Preview versions of Win 10.
But, Win 7 is the new Windows XP. Support (fixes) have stopped. There are still some security updates but that will end at some point (before 2020). Like XP, or whatever older version of Windows, it will still work after 2020.
My main PC went down, Jan 2014 for warranty repair of the motherboard. I used my backup system that has Windows 8.1 on it. I never "missed a beat" either with Sonar projects or regular PC operations. I'll be upgrading that system to Win 10 since my testing of Sonar Platinum and my recording hardware all works with the Preview versions of Win 10.
Its been reported (I think on this forum) that Win 10 will have some advantages for audio and even lower latency. Exactly what the "advantages" are, I don't know.
It looks to me that the PC version of Win 10 is getting a free ride as Microsoft is updating their phone and tablets to Win 10 and like Apple and Android providing the new version free. Microsoft would like to have one current OS across all platforms.