Unladen Swallow got a lot of hype but was only a very small project. IIRC the only people working on it were two interns.

V8 was a much higher priority - Google hired many of the world’s best VM engineers to develop it.

Some of them like Lars Bak, have background up to Self VM, which is a language much more dynamic than Python.

Anything goes regarding changing object shapes, it is one step further than Smalltalk in language plasticity.