[pygy_]

Indeed (regarding your first point), my bad. I hadn't read the complete proposal, just the SaneScript slides, where that point was not fully developed. From TFA, emphasis is mine:

However, a mode directive has the significant advantage that any program +not hitting any of the strong mode restriction+ should run unchanged in a VM not recognising the directive, and no translation step should be required.

Your next two points are different: the standard library additions can be polyfilled and the syntax change is intentionally backwards incompatible.