If the above scenario sounds unrealistic to you, that's right. It is unrealistic, just like the one you painted.
I'd rather not tell Google everything about myself but I'm honestly shocked that such extreme examples are being used to argue against the idea. By invoking Nazi analogies to make your point, you're actually doing a disservice to your argument. The instinct of my irrational mind is to think: well if it needs another Hitler to make it really scary, then I can probably ignore it.
I just finished reading the NYT best-seller In the Garden of Beasts: Love, Terror, and an American Family in Hitler's Berlin [1]. It made me think that, notwithstanding Godwin's Law [2], citizens should continue to be mindful of what can happen in a 'civilized' society.
[1] http://www.amazon.com/Garden-Beasts-Terror-American-Hitlers/... (not an affiliate link).
My plan to deal with this is to keep extremely aware of the political evolution where I live and to make sure that if it looks like such a regime is about to take power, I'm on the other side of the world with my family when that happens.
What's your plan?
If you don't want "the next Hitler" you need your political system to be designed to avoid allowing "him" to happen. Communism largely failed as a system because it consistently brought out the worst dictators in each country and put them on top. Having all this data in one place has no historical precedent. It is only smart to at least consider the worst case, given the potential damage.
If you don't like the Hitler analogy, then consider something close to home:
What would the Taliban have done with all this data on their people? The answer seems pretty obvious.
With everything linked onto one site, all you need is for that future oppressive government to have access to Google's archives to know everything about you - whether it was in the form of a purchase, a search, an email, a calendar item, a shared document subsequently deleted, a visit on a site with doubleclick or google ads, etc.
It's a totalitarian regime's wet dream come true.
"I don't like X anymore, give me all the dirt on him." "Here you go."
Or even, criticising the bundling together of any multiple services:
"The town of X is too libertarian. Send everyone a file outlining the illegal things each of them has done over the last 20 years online which would make them liable to immediate internment."
Even just the fact that google owns AdSense could be a problem from this point of view:
"Round up all those who ever arrived on a google-monitored page via hacker-news links. People from that place are 98% more likely to be subversive. I want them all shot."
It's not paranoia if they really are out to get you. And history shows that sometimes, they are.
No single entity should be made so powerful, the temptation to abuse it is just too high.
Five years after that, they make homosexual practices retroactively illegal. They look through your profile and found evidence of gay activity. You go to the internment camp, never to be seen again.
If the above scenario sounds unrealistic to you, you haven't studied history well enough.
To expand the topic a bit, imagine a government like Nazi Germany or any of the communist dictatorships, but with all this information at their disposal. The Red Khmers mass-murdered people who wore glasses, on the basis that they were intellectuals. Imagine such a regime with the tools to figure out, via mass, automated profiling, exactly who all the potentially subversive elements might be, and execute them all ahead of time, leaving a population consisting mostly of nice obedient types who never read any other book than the Da Vinci Code.