They had no choice; if GILD hadn’t tested an 8-week regimen in at least one phase-3 trial, the backlash from patient advocacy groups and third-party payers would’ve been severe and likely caused economic harm.
Just because it's a 8-week regimen doesn't guarantee advocacy groups will be happy with the price.
The interesting thing is that if what you say is true and they go with 12 week as far as pricing and what's in the label, a lot of people will use it for 8, so knowing this they will need to go in with a higher price which opens the door wider for price competition from ABBV and mix-and-match regimens.