A full task-isolation mode for the kernel

Also, why use SIGKILL, and then provide a back-door way to change it to some other signal, instead of using one of the numerous other defaults-to-terminate, or even defaults-to ignore, signals? That seems like complexity for the sake of complexity. Even inventing a new signal number for the purpose would be simpler.

Using a defaults-to-ignore signal would be more compatible with an eventual goal of automatic task isolation for programs that spin. If you want to drop core if your program violates isolation, a handler is the right way to make it happen. We don't need another.