As I learned the B747-8I is intended to bring maximum improvement over the B747-400 with minimum cost. The basic structure remains unchanged but gets a new profile (supercritical), new high lift devices (replacing the 1960s stuff) and new wing tips. Some strenghening of course due to higher take-off weight and heavier engines.
Add new engines and a slightly enlarged fuselage and you have the -8F/I. Although Boeing and its supporters sell us the B747-8 as huge step, it is a warmed over minimum cost solution. Accordingly cargo airlines are the prime customers. But don’t think that old means bad: the B747-8F practically killed the A380F as it was planned originally. Airbus will come back with a new A380F in a few years.

The 748 Is a pure ” economics ” Program , Boeing calculated the size of the VLA market , then factored in the ratio of 380 to 744 that they thought would exist , then calculated the difference in the ratio that they could create for them if they spent the 3-4 billion $ to develop the 748 and came to a conclusion that they could make a healthy ROI while keeping its existing customers happy , putting pricing pressure on the 380 etc etc . The 748I has a virtual monopoly in the VLA freight market and will pretty much have that monopoly until airbus relaunches the F 380 and even then it should compete nicely , moreover the sales of 380 PAX mean that airbus would most likely not be able to provide for the entire market demand for the Freight VLA market even post 2013-2014 , the 748 I will sell , 20 here , 10 there etc and will make profit on routes where it is suitable..the F will be the profit maker..I wouldnt be surprised if boeing sell 300-400 748’s before they call it a day , which given the investment is an awsome ROI . The 748 also gives boeing good breathing room to decide on the Y3 .