If we're looking at kernels, there have clearly been substantially rewritten kernels that launched in the 90s; they succeeded by maintaining backwards compatibi

If we're looking at kernels, there have clearly been substantially rewritten kernels that launched in the 90s; they succeeded by maintaining backwards compatibility with userland software written in the 70s/80s. If we're looking at userland, the only successful general-purpose operating systems in the last 30 years that started from scratch with no apps at all are iOS and Android. (Even there, arguably their killer app was backwards compatibility with the desktop web.) All others, including Windows, OS X, and Linux, had solid backwards compatibility facilities supporting software written for DOS, System 9, and Unix, respectively.