Boeing will sell the customers whichever model they want, and will negotiate the price individually on each. The -8 is only the least profitable if customers will pay enough more for the -9 to make up for its higher manufacturing cost.

Your example of the handful of extra 767's they sold due to the delays is irrelevant. The 787 wasn't available, and Boeing was desperately trying to fix that (so desperately they were tripping over their own feet, even). The pressure to try to salvage as much of the schedule as possible cost Boeing billions of dollars, and they did it because they knew the customers didn't want 767's as a substitute, and would even take Boeing to court if they had to.