I rather suspect that DL's SDC rules prevent it from being a revenue drain:

* Can't change from connection to nonstop (but can change connection point)
* Most Y tickets require availability in the original fare class to make the change (there doesn't need to be an available fare), so encourages booking higher fare classes
* The premium domestic fares waive the class availability requirement: Comfort+ can SDC into the last Y seat and F can SDC into any available seat... many who are planning to try to SDC will buy up to a higher fare as a gamble on there being a seat.