That is not true. The CM must do it. Which means they must know it is possible to be able to do it AND know how to do it. There are many, many posts on here--and I have personal experience--where a CM does not know one or both of those things. Including managers. That is problem number 1 in my eyes.
Problem 2 is Disney does not know when the guest purchased the ticket. Only the reseller. Even if your assumptions about UT is correct, which I'm not sure it is, there are many, many resellers out there who may have much lower stock movement. I highly doubt tickets are being returned by all with new tickets being purchased. It just doesn't make sense to me. I've worked retail. We did not return old stock in exchange for the exact same new stock simply because it was in our inventory. That is a lot of work for the seller and the supplier. You could be right, but I don't see it as reasonable. You could very well lose money if the ticket you happen to receive is much older. Many resellers will raise prices on a certain date, old or new inventory. I've often seen where resellers advertise they are keeping their old prices even though Disney raised theirs until they run out of inventory OR a certain date, whichever comes first. I think that may very well support what I'm guessing. That, and when I've upgraded to APs, I've had incorrect expert ion dates (always corrected) that are based on ticket sales. It's very possible to lose money based on frequent price changes and massive jumps at times. It's not always just about the bridging to save more.
Problem 3, no matter how well thought pity I think I have something, I've had my plans change. Length of trips have increased or trips have been added.
In the midst of this, it is no longer worth it to upgrade to APs to take additional trips. It's a much bigger cost than I was planning on. That changes other things. And now I have bigger tickets than I will be using on this trip. I had various tickets purchased/left over that I will need to fully replace. That is all more money out of Disney's pocket. I look at each bottom line.