Turns out, Getting rid of 3rd party cookies concentrates power in the ad world in the hands of those with the most 1st party data (and active session cookies). Google, Facebook, Amazon and Apple. [2]
The tracking debate is contentious and has a lot of folks on here who are privacy maximalists, and I'm not trying to debate the ethics of ads.
But in terms of utility, 3rd party cookies are the backbone of ad-tech auctions and targeting, and currently give the non-Goog/FB/Amazon ad providers a way to compete on performance advertising.
Digital ad markets are expected to be nearly $565b this year, and the Tri-opoly of Google/FB/Amazon have 68% of the market. 32% of the market depends on 3rd party cookies to have a chance at competing. [3]
1 - https://privacysandbox.com/intl/en_us/open-web/#how-works-on... 2 - https://www.siliconrepublic.com/business/googles-privacy-san... 3 - https://www.emarketer.com/content/google-facebook-amazon-acc...