I wholeheartedly disagree with several of the points you make here, and I think you're more or less ‘passing the buck’ on something which is most definitely your responsibility to take good care with.
On OS X, once you save a password using Safari, it is added to your login keychain. In order to then see that password* you must enter your login password again, be that via Safari's preferences dialog or the Keychain Access application. What Chrome does is it uses the same mechanism of storage as Safari (and indeed, requests access to the same encrypted keychain item) but never prompts the user for the login password to authenticate. This sets up a situation where Chrome actually circumvents and makes passwords originally stored in Safari less secure than they were initially. This is your responsibility, and no amount of theoretical grandstanding about physical access to a computer changes that.
You also talk about installing malicious browser extensions as another potential vector. I agree, and I think browser makers ought to take steps to require the current user to authenticate in order to install browser extensions. When you consider that a browser extension can act as man-in-the-middle to all your browser activity, it's astonishing that this isn't already the case. You have to prove you are the logged-in user when you change your password by providing the current password, so how is this any different?
When talking about security, there will always be holes, but they will be of differing practical value or risk depending on how they are exploited. Grabbing session cookies or internet history requires a certain level of technical proficiency, as does developing a malicious browser extension; installing said extension or typing chrome://settings/password into the address bar, on other hand, are easy enough that any kid who wants to get hold of his big sister's Facebook password can give it a go. Reducing the surface area of attack is every bit as important as ‘real’ security, here, and the stance you've set out above is that you aren't interested in that if it doesn't also provide real security. I think that's the wrong thing to do, and I urge you to reconsider.
*As people have pointed out, you can inspect the password via web inspector etc. This is another, serious security flaw and one that I think the HTML WG ought to look into.