Welcome to the world of airline pricing. Unfortunately I don't think Delta is in the wrong here, for a few reasons:
- Simply "cancelling" the MSP/DLH leg is not as simple as that. You are changing your flights from SFO/MSP/DLH to SFO/MSP -- your destination is changing and that is a different ticket. Remember that per contract of carriage, is MSP is just your layover, and if IRROPs happened, Delta has no responsibility to route you through MSP -- only to get you to DLH.
- It doesn't matter that DL can "resell" her seat -- there is a change being made and the rules are you must pay a $150 change fee (or whatever fare rules govern your ticket) since you are changing the ticket.
- The increased price in miles is unfortunately how it works nowadays -- airfare prices tend to trend up as departure gets closer, and now award prices do too, so you cannot demand that your ticket (purchased before T-4 days) should be the same price as the ticket price at T-4 days.