And at the end of the day, the web page is rendered on hardware (a display) owned by the user. In the very worst case, the final bitmap being displayed could be modified to remove ads; this crude approach could not be stopped short of a totalitarian state (Google isn't quite that strong) or mangling web pages into unreadability (in which case they wouldn't get traffic so ads would be pointless).
But it would never get that far, for the simple reason of accessibility (i.e. for people with disabilities). Accessible websites are machine comprehensible, to a greater or lesser degree. What the machine can comprehend (in form, if not meaning), it can edit.
During those 30 minutes of vulnerability Google is losing only the marginal cost of serving an ad-free search, a rounding error.
During the other 30 minutes ad-blockers will lose users. Users of ad-blockers are not fanatics, they have no moral issues with ads. They are normal people who are using ad-blockers because it improves their browsing experience. So what happens when these ad-blockers temporarily ruin the browsing experience? They'll temporarily turn off said ad-blocker. By the 87th time what percentage will have given up on ad-blockers for good? If that percentage is anything greater than 0% then Google is winning.
The race is not like the DRM arms race, it is more akin to the virus arms race. Users want to run untrusted code and said untrusted code wants to do something the users do not want it to do.
I would say that the arms race is highly asymmetrical against Google. Building ad-blocking technology isn't that hard, so the labor force force is significantly larger than Google could ever afford to combat.
Google is smart to not take on this battle. Only a small percentage of user's install ad-blocking software, and those users would not likely click on ads anyway. Now, if only Google could teach the RIAA of its ways.
Google search results are different every time, and the freshness of a result is essential to its utility. If you always needed "today's version of The Movie" then piracy would be much easier to thwart.
Google can have a team of 10 working full time racing Ad-Blocker's volunteers.
Google can ask for ever more aggressive permissions from web masters, Ad-Blockers must play nice with browsers' plugin api.
Google can hire away the most productive Ad-Blocker developers.
Google can ban ad-blockers from chrome's plugin repository and halve the ad-blocker userbase overnight.
Google has the warchest for a multi year engagement. Most plugin authors cannot last a single year fighting full time.
If Google went total war then ad-blockers would disappear within a year. It would be a massive PR hit, maybe even a legal issue. The risks make it all a numbers game, but there does exist a point where Google can no longer ignore ad-blockers. Thus ad-block users must be careful and not become too numerous.