You're completely ignoring the staying power of defaults. Most users do not change anything about their system defaults.

Small percent of sites will start breaking down in Safari, making small percent of users switch to Chrome. Process will start slow but will only go in one direction, and accelerate.

Apart from ChromeOS, most desktop operating systems default to other browsers. Yet Chrome is estimated to have ~70% of desktop browser market share. So defaults can't be much of a defense against browser advertising, especially on Google-owned properties.