If you want to pick nits, many C and C++ programs and OS kernels also rely on technically undefined and implementation/machine-specific behavior. From which I

If you want to pick nits, many C and C++ programs and OS kernels also rely on technically undefined and implementation/machine-specific behavior. From which I suppose we can conclude that C/C++ without undefined and/or implementation-specific behavior has been found to be unsuitable for systems programming. Or that systems programs tend to be non-portable by nature. Perhaps a more charitable conclusion is that languages like Wirth Pascal and K&R C (for example) contain (sometimes explicit and intentional) loopholes and unsafe/technically undefined behavior which can be useful for systems programming but problematic for memory safety and security/reliability. Sadly Pascal standardization seems to have stalled after ISO Extended Pascal in 1991, while C is still evolving into C23.