I suspect that this accounts for the vast majority of J seats occupied system-wide. There might be some markets where this differs, but any market that can't drive paid, corporate BE purchases would have to have some absolutely killer Y loads/yields to last very long.

Myself, I have paid out of pocket for J fares and have paid for M fares several times. I probably purchase into J, through one form or another, on maybe 60-70% of my long-haul trips. Not all of that, however, is on DL.