Nerd evangelism might not get something to 100%, but it can make something large enough to matter. Linux on the desktop matters, for example, because nerds like it and talk about it. The fact that some single-digit percentage of people use it means that the duopoly doesn't have absolute power.

Firefox, even in single-digit percentages, affects the ecosystem in the same way. If there weren't a choice, the browser would gradually turn into a cable TV interface.