"Eventually, phones will be so powerful that we'll just run our normal Linux/BSD distros* on them..."
The limitations of netbooks are not just a question of power. Windows, OS-X and desktop class Linux distributions like Ubuntu tend to require larger screens and full keyboards. That leads to a cramped user interface with small keys and a lot of side scrolling.
If we are talking about a device that has a clamshell or tablet design and a screen larger than that of a smartphone while smaller than that of a notebook, I think a mobile operating system would be more suitable.
The kind of users Google are most likely aiming this at, the ones who just use whatever browser that comes pre-installed on their new computer, are unlikely to install any extensions (including ad-blockers).
Tech savvy users who wants to block ads could always install Firefox on their computers, so I don't think allowing adblockers on Chrome would be of much detriment to Google.
Marriage is the sole cause of divorce.