I've always believed the reason why some OS's were unstable was
    because some OS's allow application developers to replace
    libraries critical to the OS (Windows is a prime example of this),
    whereas other more stable OS's (i.e., Linux) discourage the
    modification of critical/kernel OS code by applications.

Well, RedHat Linux, and maybe others, has a package manager (RPM,
RedHat Package Manager) that is aware of installed packages and the
packages have version information. So perhaps some OS allow library
management better than others.