Browsers completely stop the page from loading and show a full-page error for a certificate error. There's a huge difference between that and a small icon in the location bar. Plus, if the red icon that is suggested in that bug report is too loud, a less loud indicator could be used instead.

There's a lot more to designing a UI for this than just changing the icon. For example, when you type "foo.com" into the address bar, browsers generally default to "http://foo.com" instead of "https://foo.com." And, consequently, browsers tend to show "http://foo.com" as "foo.com" to suggest that you don't need to type the "http://" part. All of that needs to change to become a lot smarter in order for this type of idea to succeed. However, it doesn't seem completely unreasonable to consider making all those changes. In fact, I think these changes should be a high priority for browsers makers.

tl;dr: I suggest people brainstorm ways to improve upon the idea to make it workable, instead of trying to shoot it down.