For business, I fly mostly Y or B fares. Although I have total discretion over what I spend for business travel I simply cannot justify spending what all carriers, not just DL, charge for business class to Europe. Business fares to Asia are about half the cost of fares to Europe and are therefore much more acceptable to clients who ultimately end up footing the bill.

Resigned to using miles for upgrades, DL is simply not attractive anymore. Even if I were willing to burn 20k miles each way to Europe, recently revealed capacity controls (15 seats for upgrades) severely limit my chances in the BE lottery. With UA, I can upgrade a Y or B fare ticket with 10k miles each way. Moreover, I don't have to compete for seats with full fare C/J passengers as they're using miles and certs to upgrade to F. UA non-revs also sit in F freeing up even more seats for upgrades in business.

As far as the "non-rev day of flight" theory goes, DL employee friends have told me how it's much easier now to upgrade internationally than it is domestically.