This exception or assertion is triggered by the bug in CPython or in an extension. CPython developer uses release and debug builds of CPython and can get both exception or assertion. An extension developer usually uses release build of CPython with release and debug builds of an extension and can't get an assertion in CPython. Common Python user uses release builds of CPython and extensions and can only report about an exception. So this assertion can be used only in CPython developing and doesn't help to catch a bug in extensions. But an assertion itself provides less information than an exception. Debug build is less informative than release build.

May be add a runtime flag to control the reaction on system errors? If it set, all raised SystemError will be converted to fatal errors.