Whether an OS is “mainstream” anymore is rapidly becoming irrelevant in a world where most mainstream computer users are doing their most sophisticated computing with smartphones. You can make Linux or FreeBSD or even Windows work as a desktop OS, but Windows has almost all of the commercial software out there. Even then, most mainstream users are usually just web browsing, and that can be done with any OS that can run a decent (and for the most part free) web browser. The infrastructure that provides content for your web browser can be just about any decent OS, not necessarily a “mainstream” one.
Besides, an OS is useless if you can't find drivers for your hardware. Linux was lame just a few of years ago but is now pretty well supported.
Right now Windows is out in the lead. Mac is in second. Linux is third. Unix in its various forms is probably in there somewhere with BSD in its raw form?... in last place.
Also, until they improve the UI for smart phones, a larger format will always necessary. I have an Android phone but by far prefer typing on this blog with my laptop. My thumbs are too fat! The Ipad is an attempt at a compromise. But it is still a compromise. Nothing like a full keyboard.