The United States aviation scene was a totally different beast in those days. At that time, it was profitable and almost essential for major airlines to run dedicated outsize freighter services. When the 747s arrived with their cavernous underfloor holds, along with the early generations of all-cargo express carriers, these services became redundant and were dropped.
Since that time, Delta has had countless profitable quarters, so I don't think your argument holds any water.
If everyone followed the model that you speak of, long-haul flights, major hubs, and multiple fleet types would be nonexistent. We all know that Southwest's model is profitable and works well for the way they run their operation.
In this industry, much like many others, there is no panacea to cure everyone's problems. To be successful, you've got to identify your market, put out a product that appeals to the market, and deliver it on a consistent basis. How you define your market, product, and delivery is always up for interpretation, and that's what makes this industry great, almost a microcosm of the human race; so many different products, approaches, and methods all vying for different pieces of the same huge pie.
Southwest does it's job spectacularly, but there will always be room in the industry for full-service network carriers like Delta.