Talking about the company's choice to have so many different aircraft types, here is what SD says in his latest update. (Emphasis mine)

"Delta’s fleet strategy is unique in the airline industry, and our ability to efficiently integrate and operate almost any aircraft type gives us tremendous leverage in the aircraft marketplace."

"Aircraft are costly assets. Delta’s Fleet Team spends countless hours analyzing what’s in the company’s best interest when it comes to which aircraft should be in our fleet."

The company is making a very conscious choice to run many fleets. Any pilot inefficiency is due to management's choice to run an inefficient fleet make up.