“A computing monoculture is a danger, a security danger, a national security danger. It is a danger on principle. It is a danger in practice. It is avoidable and mitigable, but it is neither cheap nor easy to do so if you have to begin from where we are now.”

This text is 20 years old now, but it feels particularly relevant today:
https://www.usenix.org/legacy/event/usenix04/tech/sigs/geer.txt