I'm pretty sure that they have decided that backwards-compat is not the best path for Mojo. Matter of fact, the following is the _last_ item on the roadmap on the home page:
> Supporting more of Python's dynamic features like classes, inheritance, and untyped variables to maximize compatibility with Python code.
What's more, note how it says "to maximize compatibility" not "to achieve full compatibility."
> Supporting more of Python's dynamic features like classes, inheritance, and untyped variables to maximize compatibility with Python code.
What's more, note how it says "to maximize compatibility" not "to achieve full compatibility."