Given that most successful malware infected via a good browser (like Chrome) is obtained due by downloading (rather than an exploit) I'd say allowing top navigation is a pretty big concern. But if there is an exploit that targets a specific browser there is very little that anyone can do in advance if it is being served on an ad network.

So, if as a normal user (someone that doesn't understand malware versus OS-level dialog) sees a big warning in their trusted browser (or a popup) without having clicked on anything at all they might be compelled to download some malware. So yes, getting sandboxed iframes right would have been amazing but in my mind it was a failure.