Whether I agree with that or not depends on what you mean by "once Quiz is integrated into the activity grading interface".
How good is that integration going to be? If I am a teacher, reviewing one student's quiz attempt, then will I be one click away from grading (or editing the grade of) the essay response I am currently looking at?
Similarly, if I am looking at an assignment submission, in the context of the assignment module UI, will it be easy for me to grade it?
Also, note that what gets manually graded is the essay question attempt, not the quiz attempt.
Of course we should not maintain two grading interfaces long-term. However, Moodle is a modular system. Any integration between the activity grading system and a specific plug-in like the quiz, needs to be in mod_quiz.