With the diversity of systems running Windows, no realistic amount of testing will ever completely guarantee security updates are good. You still need a mechanism to decline known-flawed ones, and a mechanism for recovery and uninstallation the first time you get hit without warning.
In any case, the way Microsoft is going under Nadella, sadly it seems very unlikely they would do as you suggest. They are literally giving Windows 10 away free to huge numbers of people, and presumably they're going it because they want to be more like an Apple or a Google, picking up the revenues on the surrounding ecosystem, not just whatever they can find from the platform itself.
Those automatic updates would be the perfect way to show unavoidable nag messages to sign up for other Microsoft software and services, or those of their selected partners who they believe may be of interest to you, or to install spyware to feed back extra data, or to disable existing Windows feature that used to be free because some commercial interest makes getting you to pay for it a more promising option for them.
Not that I'm suggesting they'd ever do that sort of thing deliberately, of course. Maybe the Windows 7 update that has been nagging users about updating to Windows 10 itself was just an oversight.