I wonder if AC (and other carriers) might be able to see a correlation between overpriced J tickets and empty J cabins. Not very long ago (around 1994, I think) I paid $1300 for a YYZ-SFO-YYZ J ticket. Since then, this fare has risen to nearly $4000.
It simply isn't worth the money to sit in the J cabin of an A320 that has marginally more room than an economy seat.