Air Canada's B744C's have a very unusual history ...
They were originally ordered to fly on the then new Toronto/Vancouver-London-Delhi-Singapore route. The L1011-500s currently flying the route did not have sufficient freight capacity, and nothing else in the fleet had the range in extreme weather/temperature conditions.
While Air Canada and Boeing were finalizing the new aircraft, South African Airways lost an aircraft over the Indian Ocean. It was suspected, then later confirmed that an uncontrollable fire in the main deck cargo hold of the B747 Combi was the cause. One of the most effective ways to fight a fire of this sort is to starve it of oxygen, then blow a fire retardant gas into the area. This is how a fire in a lower hold would be fought, it is very effective. As you can imagine, this is not possible in a main deck hold where the air conditioning/pressurization is shared with passengers.
As a result of this accident, Transport Canada was drawing up legislation not allowing main deck cargo/passenger aircraft where this type of fire can not be fought. ie, any Combi aircraft.
Boeing and Air Canada therefore devised an aircraft with a fixed wall between the main deck cargo and passengers. This wall is a part of the airframe, is fire/heat/gas proof. Also the fire detection and suppression equipment and methods of the lower hold is now possible as this main deck hold now had its own controllable methods of air conditioning and pressurization. This was all very heavy, very inefficient and unique to those 3 aircraft ....but safe!
While all this was happening, and in response to the proposed "anti-Combi" legislation, Canadi>n Airlines announced it would suspend all Arctic flying with its Combi B737s. They decided that all passenger or all cargo aircraft would not be viable. First Air also queried how this vital link could be maintained without its own B727-100 combis.
Transport Canada then had to back down, as the government of Canada was not willing to subsidize the Arctic flying and the locals required it. Also, it was pointed out, that no other country's aviation authority was going to outlaw combi's! Instead, far stricter handling methods have been devised for carrying "Dangerous Goods".
But it was too late for Air Canada's B744 Combis. They had already passed a point in construction that could not reverse these unusual characteristics. While some unnecessary equipment was removed, the fixed, thick, heavy wall could not be removed. And if I understand correctly, this mod can not be reversed without a lot of expensive construction.
So, these orphans have remained in the fleet this long. I think if they had been able to sell then easily earlier, then they would have. As I am sure most are aware, the YYZ
route has long been abandoned.
Just because I stopped arguing, doesn't mean I think you are right. It just means I gave up!