Openness has little to do with security.
It's the whole OS design. Specifically, there's two parts: kernel and userland.
But whether an OS is open or closed is a fundamental part of the design so your explanation is a bit of a non sequitur.
It's all the stuff you run on top of it that is where people start to get screwed.
If all you ran were time-tested command line tools on top of any of those kernels, you're probably pretty well off. The attack surface area has been covered pretty well over time. Hence why Linux and OSX are typically fine.
I won't go toe-to-toe with every point you make because I agree or already understood most of what you went over, but I would point out that most of the "time-tested command line tools" in Linux and OS X are open source so you're really arguing against yourself pointing that out.
Where I disagree with you is whether or not being open contributes to software security. I think it does. One of the things I've observed over the years is that open source security issues are usually noticed and patched
before it becomes a widespread problem. That's a direct result of so many eyes looking at the code and so many programmers having access to it. Noticing the security issue and patching it becomes a proactive thing, i.e., action is taken before it becomes a problem for end users.
With Windows, it seems to be more of a reactive environment. The issue gets patched after countless machines have already been compromised and it has already become an issue for end users. Because fewer eyes are seeing the code and fewer programmers can contribute a fix, the process is a lot slower and more painful.
So, maybe we're both right but looking at it from different angles. Maybe OS X and Linux are no more secure than Windows, but because there's a larger degree of openness with them, the potential for viruses is minimized because of reaction time.
Who knows? But I think it's inaccurate to say open source software is no more secure than closed. It's pretty clear that it is in most cases.
And in the case of Android versus iOS, I'd point out that Android's jaw-droppingly poor support for OS updates probably makes security issues far worse than they should be.
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Laughing so hard right now at the ignorant idiots who keep claiming "There isn't one OS X virus right now."
Do you people understand how systems work? There are tons upon tons of viruses in existence for basically any platform you could think of. I could go and write a small virus right now and gift it you right now if I wanted too.
Viruses can be created by anyway willing to learn, and for you to say OSX is virus free is the best joke I've heard all year.
Not to defend anyone's delusions, but when most people say "virus" they are referring to Windows-like widespread outbreaks that affect numerous machines and cause problems. In that sense, there are no OS X viruses.