I believe you are the one that brought up IS as a failure point in cameras and I just went into say there are many failure points in today's camera and pros carry backup systems just for this reason. Basically IS being failure prone in cameras is no more an issue than any other component being failure prone. Definetly not a reason to not provide IS in the body.
But you are too fixated on "pro", and professionals bringing spare equipment.
The majority of people on POTN aren't professionals. The majority of Canon customers aren't professionals.
A person going on vacation might not be able to afford a spare body. And a person taking a weekend at a zoo might not want/be able to carry a redundant set of equipment.
The implication of a failed in-camera IS would matter way more than a failed lens IS for a very large number of Canon customers.
And a Canon professional would know that all longer lenses do require IS in the lens. While you haven't really managed to show that there are any significant price jump for getting IS in the wider lenses. Canon's cheapest IS lenses are about $200.



