Justice Thomas replaced Thurgood Marshall. No way anyone but a black man was going to be selected for that vacancy. So one could say that Justice Thomas is proof positive that even an affirmative action appointee can turn out to be an outstanding selection.
In fairness, there is a tradition of various seats on the Supreme Court being “tagged” that goes back to basically the beginning, though the tag varied-—a “Virginia-Maryland” seat, a “Deep South Seat”, (much later) a “Catholic seat.”
If it had been a different Republican, perhaps not. But your point is well-taken. If one does one’s very best within a group—great. Too much of this leads, however, to almost everyone within the group being misplaced.