Although I have no facts, I believe the AC sells enough seats in the front on domestic and US flights to justify the cabin and the size if the cabin. Compared to the US carriers, AC normally has one extra row in J. They wouldn't bother unless they are selling the seats. If it was just an upgrade perk, they would certainly reduce the cabin size. As a recent example, I flew YWG -YYZ last Wednesday afternoon. J=0 and R=0 at T-7. No upgrades were called at the gate. AC appeared to sell most if not all of the J seats.