I can see that, but I just always go under the assumption a single box can be compromised. My main concern, generally, is mitigation and identifying the threat as soon as possible. I agree that fixing OS problems can be more complicated and difficult to find, but Microsoft and more recently Apple have been doing a much better job then they were a decade ago.

A vast majority of the common malware threats are due to people's ignorance or laziness. You can properly harden your personal system to a reasonable point. Sure, that might mean having to whitelist javascript and be cautious about what you download/verify MD5 hashes. But, security in real life is cumbersome too.

If we're talking OS, what concerns me the most right now is the coming tablet-centric operating systems -- especially iOS4/5/Android. On the one hand, you have iOS which is extremely closed and runs on a device which in and of itself is closed/difficult to open. Then you have Android, which is going through the wild west syndrome. While Apple's AppStore is a bit draconian for my tastes, it's undeniable that stringent measures do help security if not stifle creativity.