Offering an incentive to cancel is effectively the same thing as softening the refund / change policy. That will change the breakage rate and therefore the economics of the operation. Nearly all airlines I fly are fully non-changeable, non-refundable and I get that. UA has always been remarkably generous, with Same Day Changes permitted and, against a fee, the ability to capture residual value in a ticket. However, I see that they are moving away from such generosity and towards the more common model.

Looked at by itself, the OP's suggestion makes perfect sense but, looked at in the context of the buying decision, it's a much more debatable idea.