> No it's pretty clear what are ads or not on Google.

It is for you and I, but not for the average user. My grandpa clicks on sponsorized links because they are at the top, no matter the presence of the word "ads" or "sponsored".

It would be a lot clearer not to have ads at the top but elsewhere, so that the “best” result is always at the top.

But then you would complain your grandpa is clicking on the links because they are yellow and flashy "We need a law against yellow <a> tags !"

No. We’re used to click on the top links because that has been the way to present search results since the origin of search engines. The presentation of ad links elsewhere would not trigger the same amount of misclicks, whatever their presentation would be.