Couple corrections:

- They absolutely do have to serialize, usually via pickle. I'm pretty sure objects are not sharable between subinterpreters and there is not a plan for that. The main reason people think subinterpreters are good ("you can just share the memory!") is not actually true.

- They don't require any changes to the C interface because those changes were already made, and a fair amount of cost was paid by C library maintainers. So it's true, subinterpreters are at an advantage in this regard, but that's more of a political question than a technical one