It's not a circular argument, because JavaScript is not the only reason to use a browser, and browsers are not the only reason to use JavaScript. There are so many other good arguments for writing a game in JavaScript and for using web browsers as a platform. For example, the ubiquity of the platform, the available of so many libraries for rendering graphics and implementing user interfaces (as I mentioned above, somebody's already written a WebGL library for rendering digital Lego bricks, for example), and so on.
I've already proposed (and implemented) JavaScript as an alternative to C++, just as I earlier proposed (and implemented) C++ as an alternative to C. I've ported SimCity to numerous platforms, rewritten it in C++, and written user interfaces for it in PostScript/NeWS, TCL/Tk/X11, Python/GTK and OpenLaszlo/Flash. So I have a good idea of its requirements, and I think JavaScript in a web browser is an ideal language and platform for it at this point in history. Do you disagree with any of that, and can you suggest anything more "natural"? If not, then keep quiet, and we're done here.