First off, let me correct a couple of your number.
The 787 was NOT launched with 200 committments. It was launched with just 50 from ANA!
Would they launch a plane with 200 committments? You bet your burger and fries they would! Because if any one airline were prepared to order 200+ aircraft, others would surely follow.
Boeing only wishes that they had 200 orders for several of their planes. The 747SP sold only 47. The MD
-90 sold 128. The 717 will only sell roughly 175. As far as derivatives, the 757-300 only sold about 50. The 767-400 only 'raked in' orders for 40 or so. And the latest stress maker is the 777-2LR, which only had orders for FIVE aircraft until last week!
All that said, I think that Southwest would most certainly be THE launchcustomer for this airplane. Considering that they launched the 737-second gen by placing the first orders for the -300, and later launched the -500, and went on to launch the -700, it would only fit that they launch the 797.