There are a lot of things about Plan 9 that really aren't that good in 2016. The compiler, for example, lacks a modern optimization pipeline and generates code that is unacceptably slow by modern standards. The windowing system is not a compositing window manager; it's a '90s design through and through and cannot perform as well as solutions like Wayland (that are arguably simpler). The browser doesn't support dynamic changes and would have to be rearchitected to support anything more than simple pages with acceptable performance.

None of this is meant to bash the great work that the Plan 9 team did while at Bell Labs/Lucent, mind you. With the team they had they did a really great job, and they're all top-notch developers. I only want to counter the notion that Plan 9 is all around a technically better OS than the ones we use today. It isn't.

As sad as it sounds, most complexity exists for a reason. There is a good amount of cruft that could be dropped if not for backwards compatibility, but when e.g. compilers are concerned, you depend on that complexity for performance.