Is Microsoft a National Security Threat?

Not Microsoft per se, but companies who use such an insecure OS in critical infrastructure

No. I don't like it either, but Windows is useful compared to the other listed operating systems. The graph doesn't carry a meaning, since it isn't normalized for distribution and the question doesn't answer anything about the vulnerability of a system.

10 years ago I wouldn't have imagined that I praise Windows for its openness due to general computing, but the other candidates here aren't convincing. MacOS might be an exception still, but it causes even more problems than Linux in corporate environments.

It would be nice to have some domain infrastructure for Linux, as that would be a far more secure future for private corporate networks. And not only because of the malware threat.