The fact of the matter is you can still use the value of the ticket at a later time by paying a penalty. You are using a perishable product, and holding a seat that DL
can sell to someone else. The bean counters associate a dollar amount to that lost revenue, and not charging penalties means DL
(or any other carrier) doesn't recoup the lost revenue.
Unfortunately, fees for checking bags, buying tickets at a ticket counter, and premium seats are also horrible customer service but a necessary reality today. If all airlines were to go along with a unilateral price increase to make ticket prices cover all of these things which we now consider "extras," we'd be back to appropriate pricing and we could have the frills that used to be there. I don't like it any more than you do, but it's our reality.
Compare it to travel services like Hotwire and Priceline. You get a great price but with no changes permitted. Personally, I'd rather take my chances and go with the low price.
I am a ticket agent and see so much abuse with the international ticket "medical" waivers. Parents coming into the USA to visit their children, and their 3 week trip turns into a 6 month soujurn with a vague medical letter saying the person was there for "treatment." I honestly don't think we should give a medical waiver. If the person can afford medical care in the USA, they can afford to pay the date change penalty or a fare that allows a longer maxium stay. I've seen this abuse all the time.