Google's an AI company. If they are guessing wrong, then they're doing poorly as a company and will suffer financially. They have every incentive to make sure it guesses wrong as infrequently as possible. You have many interests, but a finite number, they can all play a role in the model of you. Similarly with people you know.
Now, if you're suggesting that for some reason there's an inherent "you will never be able to predict me with >x% accuracy", that's another topic.
I'm not even one person. We have many computers in our home; I'm logged in to all of them, Google-wise, but they are used by my wife, the woman who looks after our kids, our children themselves, etc.
It would be very impractical to have each person log in as herself before they can search anything on Google (and my kids are too young to even be allowed to have a Google account!)
For now, customization seems to be attached only to the computer (browser), not the account; I have a computer that I use for work that no one else has access to, where Google searches work fine; I practically can't use the other computers to search Google.
If some day in the future, results are customized according to the account instead of simply the browser, then I won't be able to use Google at all.
> Google's an AI company
Ok, fine. Here's what they could do, then: detect topics in search results (and in query terms) and let one cluster or filter results by topics / domains.
They do - for selected queries where it makes sense.
Try searching for "jaguar" (no quotes). On the right, I have "Searches related to jaguar", which include "jaguar big cat" "jaguar car" and "jaguar download"
This. If they were actually guessing wrong, they would just stop guessing. Even if they didn't employ mostly geniuses, they would know to compare their models against random chance.
But my guess is, their models will actually do considerably better than random chance. I find google searches massively more helpful than duckduckgo searches not because ddg isn't a good search engine, but because ddg isn's snooping on me and just doesn't already know that I work a lot with such-and-such CMS.
And this is really all much ado about nothing, since if google were to ever overfit and render me unable to find something that it thinks is not my usual interest, I would just browse right back over to ddg.
Now, if you're suggesting that for some reason there's an inherent "you will never be able to predict me with >x% accuracy", that's another topic.