Isn't this also a method pay-walled sites use to get subscribers? Otherwise, why not just block Google in robots.txt?

They still want to be found. They just also want to be paid for the privilege of being listed on a search result page.

Google should just start putting a huge red box around pay walled articles. It would basically indicate "don't bother with this site"

As a user, i would actually love if Google had that feature natively. I assume there is a way to have that functionality already by using some browser extensions. But a native implementation by Google would be a killer feature imo.

Implementing it would essentially require the browser knowing if you have logins/subscriptions. Taken further they could personalize searches to only include paywalled sites if you have a login as opposed to marking them as "don't bother".

Google would only require 2 things. If a site has a paywallz and if the user whitelisted it. I have black listed most paywall sites manually on my Google feed. So they are obviously capable and already doing the latter.

And the blacklisting is easy. A click of the button and it's off my list