The _PyMainInterpreterConfig structure is redundant with _PyCoreConfig: it is a subset but using PyObject*. PyInterpreterState has 2 fields:

    _PyCoreConfig core_config;
    _PyMainInterpreterConfig config;

config parameters can be found in core_config, but using a different type: it introduces redundancy and a risk of inconsitency (if one is modified, but not the other).

_PyMainInterpreterConfig doesn't provide any feature which is not accessible using _PyCoreConfig.