The "pluggability of Perl at all levels" is a goal meant to move us away from the existing Perl 5 core design, which is mostly C macros and is essentially unmaintainable. Even if we never truly achieve pluggability on ALL levels, we must at least work TOWARD that goal, in order to achieve the other big plans such as reintegration of Perl 5 + Perl 6, and C/C++ performance, etc.