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).
I imagine they'll charge money. Mighty's $30/month is a bit out of my price range, but given that I spend ~70% of the workday in a browser, if one could improve my productivity the amount I'd pay for it isn't nothing.
I'd argue that IE6 got worse and didn't keep up with developments while Firefox got consistently better. Firefox' problem as "only" that is was really slow and that's what Chrome solved.
Yet, I also think that a new UI doesn't really change the browser experience much. It must be a completely different UX.
Firefox came at a time when the web was really annoying to use, and had features to make it better. I don't think IE had even shipped a pop-up blocker at the time. But FireFox's extensibility also made things like ad blockers and Grease Monkey scripts possible, which was the only way to stay sane as a power user in that era. And tabs! That was a big one.
I think the web has gotten annoying again, in different ways, because of the thousand things that nag you on every website to click to close them or otherwise obstruct your reading (cookie banners, too). If a browser can solve that reliably, I'd switch to it.
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).