Parking spots is the issue. I’m sure at some point they’ll figure it out, but right now you can fit 600000lbs of md 11 payload in 420000lbs of 77f spots. Also they’d have to redo entire sections on the mem ramp. Not insurmountable, esp if fuel gets to be an issue again, or a permanent grounding, but that’s what has kept the 777 from being a one for one replacement in the past.