Exactly. But it's not neccesary a bad thing though.
What all the "smaller players" in emerging retracking field - where 3rd party cookies are used in the first place - are doing now is nothing conceptually different from "ah, you've added iPhone to shopping cart at shopX! Now we'll show you iPhone ads for a week on every site you visit!". And user is beating her head against the wall because she bought the iPhone offile a week ago.
If 3rd party cookies will be disabled by default in significant part of browsers, all that players will need to find a ways to add some value visible to users and show it to them and convince them to enable 3rd party cookies manually because that will be just the only way to track them.
We don't know yet what that added value(s) could be, but I'm sure they will be invented. And that would be good thing.