All of the popular full, un-sandboxed operating systems for end-users are proprietary. No, desktop GNU/Linux doesn't count. The only non-proprietary platform that has sewrious user market-share and developer mind-share is the Web platform. Therefore, as much as it may disgust us from a technical/aesthetic point of view, advancing the open Web platform is the best way to achieve certain highly desirable social/political goals.
And by "all of the popular full, un-sandboxed operating systems for end-users" you mean the one that isn't sandboxed, Windows? because, technically even just the popular part could mean only Windows, but the addition of un-sandboxed further cements that because OS X sandboxes things a bunch now.
