The problem is that intustrial installations of the type stuxnet targets usually cannot be patched at all because changing anything important in a system like t

The problem is that intustrial installations of the type stuxnet targets usually cannot be patched at all because changing anything important in a system like that requires a full reevaluation of compliancee with all current security and other regulations as if the installation were new. And this can easily cost tens or even hundreds of millions in hardware changes if the requirements for new installations have become stricter since when the installation was built (which is quite common for nuclear facilities).