It is all about keeping the OS sane to give the users and applications a chance to be sane, rather than causing madness from the get-go.
However, if the system permits arbitrary applications to be executed (which plan9 does), then it cannot be shielded from madness. Proof: plan9 has a POSIX compatibility layer (APE), a vt100 terminal emulator and a vim port. It even has an aging full-on linux emulator. I think it could run a really old version of Firefox, but the memory is vague. People swear by Mothra these days anyway.
However, if the system permits arbitrary applications to be executed (which plan9 does), then it cannot be shielded from madness. Proof: plan9 has a POSIX compatibility layer (APE), a vt100 terminal emulator and a vim port. It even has an aging full-on linux emulator. I think it could run a really old version of Firefox, but the memory is vague. People swear by Mothra these days anyway.