When these were first written, it was assumed that the classes would be used purely from C++. However, bindings to environments like Python can be easily written against the C++ API. But the classes being final makes it hard to add environment specific functionality. Bindings authors could use composition but that requires all callsites to annotated differently which makes the whole process more tedious.

The C++ classes are also just wrappers over a handle to the underlying Impeller type.