I've flown on DL (and WN) at STL a few times recently.
DL's terminal (A, I believe) is a disgusting dump. Certainly not blaming DL for this. It would be nice to see STL upgrade a couple of those mothballed terminals to provide a better experience for passengers. WN's terminal is okay: at least has a couple of places to eat.
Happy to see DL add service to STL. Agree with the comments that LAX service would be a plus. Not sure it would be profitable, but it would be nice.