The problem with replacing the 732s with mainline a/c is what to replace them with. Your only choices at a plane for plan replacement are the 717, 736, and 318. The 736 and 318 are heavier and longer ranged that what DL
uses their 732s for (unlikely DL
would go Airbus). Further the 736 would actually more expensive from a flight crew perspective than the MD
-80 is (737NG payscale). The 717's future is in doubt. As for replacing them with E195s, this is unlikely due to the scope clause which would force them to be mainline a/c and limit their profitability. My guess is DL
will up and downsize the 732s to CRJ700s and MD
-88s when they are replaced (this assumes the 733 family will go simultaneously). This would allow 737-7/800s to be picked up to replace the MD
-88 positions to maintain capacity.
Of course give a/c age and usage DL
will have to deal with their 762s and early 763s first. They are already hangar queens
Of course all of this assumes DL
will be in a cash flow position to execute these options.