I see two options for Delta if they want to continue to offer SWUs that are anything less than a joke compared to AA or even UA.

1) Reduce the restrictions on fares and institute a co-pay system (like both AA and UA have done)

2) Manage M fare cost. Over the last year M fares have practically doubled. In 2011, I paid 2,200 for a JFK-NRT RT (a little steep compared to the competition's upgradable fares, but still manageable). This year they are actually higher than the S fares (discounted Biz) at around 5k. For a highly competitive route this just doesn't make sense.