Here's the problem.

DL designed a product and a procedure for C+ upgrades based on their experience with Y->F upgrades. Unfortunately, the C+ product doesn't deliver in the opinion of actual DL Medallion customers.

In the case of Y->F, there is little controversy that the worst F seat is actually better than the best Y seat. No-one is going to get upset with an upgrade to F.

Contrast that with Y->C+. In this scenario, the seats left for assignment at the gate are often middle seats (understandably). Unless you are DL's marketing team who only talk to each other, the limited C+ "amenities", such as they are, don't make up for the "downgrade" of moving a Medallion from their carefully chosen (lets say) exit row aisle seat to a C+ middle seat.

The hue and cry from the upset Medallions - on every flight - must have been so bad that gate agents did what anyone would logically do in the case where their company created a procedure which angered their best customers. They stopped running the C+ upgrade list.

In my opinion, many (not all, but many) gate agents stopped running C+ upgrades because their best customers on every flight were getting the most upset.

Y->C+ will work just as soon as DL creates a C+ product where the worst C+ seat is better than the best Y seat. Until then, the upgrade process simply will not work. In my opinion.