That operating system needs to be continuously updated with security fixes.
What you mean "need". The license doesn't put them in any liability for the consequences of security flaws. People don't use Windows and Office because they're the most secure products, they use Windows and Office because they are locked in. Microsoft could declare right now they're not going to fix any more security flaws in Windows, all future installations will be licensed at a fee of $1K/CPU/year, and people wouldn't have any more choice than they do now.