You probably heard this one many times: "Hard to define, but you'll know it when you see it" (A US Supreme Court judge about pornography). So there's that.
The line is different depending on the genre. If McCurry said, in a preface to books or exhibitions, something to the effect of "I spent most of my life as a documentary photographer, but this book/exhibition is my artistic vision of the same scenes, where are took the poetic license to alter what was happening in front of my lens by removing a few elements, including human, in order to better highlight the essence of the scene...blah, blah, blah" I doubt we would be having this discussion. Stupid photoshopping notwithstanding.
On a side note, that single photoshopping blunder is much more damaging because it brings into question not just that photograph, but the whole opus. The lingering question then becomes: "If he manipulated that one, which others he did as well?"
I never thought he manipulated any, as I always thought he was predominantly shooting film, slides at that.