I'm actually going to flip around and take an opposite tack here: Why doesn't DL just bin MQMs entirely and set de facto MQD thresholds, and then perhaps waive part of the requirement with CC spend?
...oh, sorry, that would be the old VX program, which made sense to me.
(TBH the MQM/MQD tangle is part of why I've taken to outsourcing my earnings to other carriers...I've got a preferred analogue carrier for each of the US carriers, and for the most part they have more suitable international networks for my purposes.)