Nobody thought anybody could dethrone IE when Firefox came around. I don't think the goal of Arc is to be the browser that everybody uses, so much to be a useful browser for a lot of power-users.

With the industry standardizing (for better or worse) on a handful of rendering engines, it's not a big stretch to imagine a world where you can choose between different browsers and still experience the same web as everybody else (as opposed to the days past where choosing a niche browser meant accepting a worse web).