The thing to remember here is that 'miles' are basically administrative fugazi or fairy dust if you wish. They are nothing but a highly sophisticated and elaborate version of a coupon in the mail for a repeat customer. The system has to be so complex because so many quite intelligent people spend time thinking about how to game this system. But at the end of the day, this is Delta saying to you, we like that you fly with us, here's a bit of an extra incentive to do it again. It's marketing, nothing else.
People who expect 'miles' to work like regular currency are delusional.