Delta want to cut there number of Diamonds in half (if not more)
The lowest value customers should be the ones they trim.
Removal of the MQD waiver achieves that goal.
That makes sense.

What doesn't make sense is making having the AMEX card a stupid decision. I don't personally care, but no one in their right mind would use a Delta Amex - when 2.625% cash back no fee cards are available.

Plus from a perception viewpoint AA's program sounds much better. Telling someone 1 MQD per $20 dollars sounds far worse than 1 loyalty point per dollar.
Telling an Elite you get 11 LP's per dollar spend sounds better
Delta should have copied AA