Third party cookies, love them or hate them, have been with us for a long time, and simply dropping them would not be viable without the long phase out. And a long phase out is not something around which you can form a singular rallying cry.
FLoC is a new thing which is just being rolled out, so it's a lot easier for people to resist adding a new thing that makes the internet more crappy and less private.
I think it's unnecessarily fatalist to say that all of this will continue to happen so what's the point of resisting it. Public awareness and negative opinion of the pervasiveness and creepiness of internet tracking continues to grow, and advocacy against tracking mechanisms helps create the type of groundswell which could actually shift public policy to forbid such tracking.
Google specifically is catching some heat for potential antitrust problems, so raising a ruckus about Google abusing its dominant browser position to cram FLoC into the internet is more likely to have positive effect than ever before.