I would argue a good 99% of what we experience in life should be opt-in and yet here we are. Use of AI, companies getting our data, etc is all opt-out meanwhile organ donation is opt-in. Wtf.

When you use the site for the first time and search something where AI overview is available, it shows a toggle for how often you want to see it, with "sometimes" as middle ground being the default. Without showing that a feature exists, it's hard to decide if you want to use it or not. And since peoples opinion about it seems pretty spit, I think that is pretty valid.

Or would you rather have a cookie-popup like choice box pop up every time you use the service without cookies enabled asking you for your preference? Because that would annoy me very much.

Folks can configure this as their homepage, search engine, etc.