Same applies for Platinums and Gold. Within a Medallion cateogory they rank the upgrade list by last year's MQMs and fare category of ticket (and probably total miles too). So a Silver on a higher fare category would be greater than a Silver on a lower fare category, and two for Silvers on equal fares the higher mileage should win.

I agree Delta should have a program to promote the retirement of miles by paying more miles to upgrade from a lower fare category, sort of like SkySaver and SkyChoice, assuming those in higher fare categories have refused to upgrade by payment or miles.

Outstanding miles are a liability on an airline's balance sheet. They should want them off.