"every system at the center of a big enterprise"...that's the thing. A lot of enterprises actually don't need this kind of uptime or can get by with other types of redundancy.
Your comment about software, though, is a good point. The last relatively common OS that has that kind of uptime is OpenVMS.
Your comment about software, though, is a good point. The last relatively common OS that has that kind of uptime is OpenVMS.