The original Win 7 eula does not include that right. Win 10 eula does.Well, there's no such guarantee, but can you be sure your beloved Win 7 or whatever doesn't have a back-door to enable M$ to de-activate it remotely if they so desired?
Ones remedy, of course, is to re-install Win 7, but by that time, M$ may no longer recognize the activation code. Almost that exact thing happened when I needed to re-install Photoshop CS3 after I had to replace the C-drive a bit more than year ago. It only took one letter to get my re-installed version re-activated. There was no communication from Adobe, it just happened when I turned on my PC one day shortly after the letter.
Unfortunately, reversion to Win 7 will have its problems, even if it can be activated. You may loose Win 10 files, e-mails, and shortcuts and folders on the Desktop. I suspect M$ will disavow any responsibility for those problems. After all, it gave you "free" use of Win 10 for however long, and it warned you to make back-ups.
John