“Customers will not receive the January 2018 security updates (or any subsequent security updates) and will not be protected from security vulnerabilities unless their antivirus software vendor sets the following registry key” Another incentive to stop using questionable AV software (since this was implemented because they can't get their act together).
No, another incentive to stop using Windows. 3rd party applications should NOT be responsible for insuring that the OS can receive critical security updates, and Microsoft should not be relying on 3rd party applications to determine whether or not their customers receive critical OS security updates (and of all things, hilariously defaulting to 'no')
Difficult situation for Microsoft. If you install applications which mess with the operating system in unsupported ways you can't expect your system to function correctly with automatic updates. On the other hand, users are likely not aware of what they have done and bricking millions of computers is also not good. Might for example cause a backslash when people stop updating their systems.
This sounds like a quick and dirty fix they put in place while figuring out what to do.