>> Sensible type-annotated python code could be so much faster if it didn't have to assume everything could change at any time.
Then it wouldn't be Python any more.
Fine by me. I don't particularly like Python, but it's the defacto standard in my field so I have to use it (admittedly this is an improvement over a decade ago, when MATLAB was the defacto standard). I don't care about preserving the spirit of Python, I just care that the thing that bears the name Python meets my needs.
I share your view. Python's flexibility is central to Python.
Even type annotations, though useful, can get in the way for certain tasks.Betting on things like these to speed up things would be a mistake, since it would kind of force you to follow that style.
Anything that accelearates things should rely on run-time data, not on type annotations that won't change.