it is a far more fixed interface that is far harder to evolve
...
Its about making VM more flexible at run-time, not at compile time.
These are exactly my recent thoughts and which led me to get reacquainted with Forth as the basis for a "re-programmable" plug-in. It's hard then not to consider a Forth based VM bootstrapped from the image itself, although I'm not yet sure this is even a sane idea :-) I wouldn't be surprised if someone has already considered and rejected it.
-D