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Alex: yes, the builtins could almost all be re-implemented in pure Python, but there is a performance consideration vs. benefit (e.g. if we re-implemented map() in Python which VMs would use it vs. implement it in some native way for performance?).
But one place I think there could be a benefit w/o performance issues is the built-in exceptions as you should be able to store references to the classes to help minimize the added cost of startup creating the exceptions from Python code.
Amaury: other VMs and easier extensibility in the future. We might as well all share the burden of maintaining pure Python versions instead of re-implementing the same thing over and over in little VM silos. And if some new thing needs to be added for a module it's easier to do it in Python than C (which then also continues to benefit other VMs).