Not sure I understand the question. If you don't check in and don't fly, why would anything other than "I get nothing" even be an option?
Cheers.
Many thanks folks, for the quick, helpful responses.
On this last remark: I was fearing that if I miss checking in the last segment, perhaps AA might forfeit the miles for all previous segments flown.
But not that you mentioned it: what if I checked in for all segments (with no checked-in bags), but still miss the last one? I don't plan to do this often, but I am considering an upcoming trip in which I want to visit a cousin in BOS and we drive to NYC together...hence my missing the last BOS-NYC segment.
I suppose checking in the last segment and not flying it means no miles for that last one.