I think it comes down to how the Chrome team is used to thinking about security. Most of Chrome's security work, such as the phishing blacklist and the process

I think it comes down to how the Chrome team is used to thinking about security. Most of Chrome's security work, such as the phishing blacklist and the process sandbox, aims to prevent attacks from malicious web pages. In those circumstances, "pretty good" security gets you nowhere. If there's a difficult but known way to break the sandbox and execute arbitrary code, it needs to be patched. Otherwise, the hole's obscurity won't prevent people from distributing exploit kits. A subtly broken sandbox is almost exactly as secure as no sandbox at all. If you're accustomed to thinking of security in this all-or-nothing way, the Chrome team's argument makes sense. And the state of the Internet encourages this kind of thinking. In the real world there are plenty of gray areas to security, but online there's always a sufficiently advanced and determined attacker.