I'd imagine DL is monitoring how much a premium and what percentage of customers are upgrading very carefully. There are people like me who recognize that ComfortPlus is truly what Main Cabin used to be 20 years ago, and therefore will never sit anywhere lower in service than comfort plus on a flight.
If they find they need to sell more F seats or C+ seats, I have no doubt they will increase their seats available. I remember a few months ago United started some sort of ad campaign about having the most premium seats on any airline within x number of years. If Delta viewed this to be in their financial interests to pursue a similar strategy, don't think for a second they wouldn't do it.
I'd love to see the 757s refitted with a full F cabin when turning left, like the NW days....