777-8/9 is at least 5-10 years off. It really depends on the A350 and if/when it challenges the 777's supremacy. Surely, Boeing will not introduce a better 777 while trying to market the -8I. They aren't that stupid. Milk the cow you have first, then get a new one.
As for the 777 only assumption, I think that is unlikely. For the sake of argument, let's assume the 777 has 300 seats and the -8I has 400 seats. The difference between operating 777 only and a combination is the difference between 3,6,9,12 hundred seat offering and 3,4,6,7,8,9,10,11,12 hundred seat offering to the market. That is a huge increase in flexibility from an aircraft the company already has (type-wise) on the property with pilots and engineering all trained up and operating.