The battle at Delta has always been between having too many fleet types and network saying they can earn plenty of money to support different fleet types. Network likes different sized airplanes to supply a market to match demand as close as they can.
Even the large 737-900ER order has had changes during the run. Example, the early airplanes had different landing lights than later planes - so just the 737-900ER fleet will have differing part numbers within the sub-fleet.