Graham, I appreciate the history of sub-interpreters and how entrenched
they are.  Changing those practises requires a significant investment. 
This is an important factor to consider.

The other factor is the continuing maintenance and development cost. 
Subinterpreters add substantial complexity, which I can personally vouch
for.  This is exhibited in the GIL API not supporting them properly and
in the various bugs that have been found over the years.

Imagine, for a moment, that the situation were reversed; that everything
were built on threading.  Would you consider even for a moment adding
sub-interpreters?  How could you justify it?

It's not a decision to be taken lightly, but my preference is clear:
bite the bullet, make the change.  It's easier in the long run.