You may consider allowing a customer with a business class ticket to sit in an available economy seat is "pandering to every whim of a customer", I certainly do not (and, frankly, I can't imagine many other people would either).
If you want to downgrade to L class then that is all fine and dandy; good customer service does not involve pandering to the whim that a person should be allowed to book on a flight regardless of whether there is availability in an appropriate booking class
The OW ATW fares are clear on what the voluntary downgrade process is; the same principle applies to a 1st class holder wanting to travel in business class; D is required, not J or C
A full fare 1st ticket is different ( but typically a lot more expensive ) in that if F is not available then J can be used and then Y
Dave