Third party cookies are not simply a matter of making adtech developer's lives easier. Imagine you visit shoes.example and are now on news.example. Both of these sites work with ads.example, and the shoe site would like to show you a shoe ad.

With third party cookies this looks like (simplified MVP form):

1. When you visited shoes.example, it loaded a pixel from ads.example. That pixel automatically sent your ads.example cookie, and put you on a remarketing list.

2. When you visit news.example, it sent an ad request to ads.example, which also automatically sent your ads.example cookie. Now the ad tech vendor knows to include the ad from the shoe site because it recognizes the third-party cookie.

On the other hand, without third-party cookies or any replacement browser APIs, how do these identities get joined? Very occasionally someone will follow a link between a pair of sites, and then you can join first party identities, but you probably don't have a chain of identities that connects a news.example first-party identity to a shoes.example identity.