One other scenario - if Y is oversold they may upgrade non-elites who paid top fare for coach, not to mention anyone purchasing full Y would be upgraded.
This is true.
I'm guessing it would just be one more headache for DL to upgrade the PAX with 22,000 MQM's this year who's travelling on a T fare if seats were available. Basically I'm talking about ridding the airline of the purchased upgrades.