Occasionally, my friend Jerricho is correct!
Mixed mode would work for twenty minutes, and then you'd get the situation of nothing at the holding points because all the aircraft are in a massive traffic jam!
Lots of other reasons why mixed monde won't work, I'll list them all if you'd like! If you purely think of it in runway terms; presently we average 80-82 an hour. In mixed mode you'd be looking at something like 90-100.
I remain convinced that in mixed mode, our GMC capacity actually decreases.