If Chrome blocked third party cookies today we'd see something between these two outcomes:
a) Publishers lose about half their revenue because ads aren't personalized anymore: https://services.google.com/fh/files/misc/disabling_third-pa...
b) Advertisers figure out how to keep personalizing ads through fingerprinting (non-cookie tracking)
Since (b) is worse than the status quo (users can't reset their fingerprint) I think "a security improvement that is literally flipping a switch" doesn't fit.
Chrome's approach (as described in https://blog.chromium.org/2020/01/building-more-private-web-...) is:
* Block fingerprinting
* Figure out how to let advertisers personalize in privacy preserving ways (https://www.chromium.org/Home/chromium-privacy/privacy-sandb... primarily FLoC and TURTLE-DOV)
* Then remove cookies
I'm skeptical about the approach, since I think blocking fingerprinting and server-side correlation of requests is very difficult, but I think the people working on this are very good and have thought a lot more about it than I have.