I seriously doubt Boeing would develop such a variant. It would require major changes to the wings and other systems on the 747-8I. The whole point of the 747-8 was that it required little development beyond adapting technologies developed for other Boeing programs like the 777 and 787 to the 747. Making a very long ranged variant would require longer and strengthened wings, heavier landing gear, bigger engines, and a strengthened fuselage to carry the additional weight of more fuel. The engines may become available if Boeing builds a 787-10X stretched version. I doubt there would be much benefit to Boeing in spending the other money necessary to build such a long ranged variant of the 747 when there would be very small demand for it.
The 777-200LR shares much of its development costs with the 777-300ER and 777-200F. Those planes will probably be built in 500+ quantities over 10-20 years. It could even be more if some of the additional air force tankers are based on the 777-200F. The 747-8 freighter and intercontinental models are expected to sell 200-400 airframes over the next 20 years. I don't see how a 10,000nm ranged 747 would sell enough copies to make economic sense. Perhaps an even larger stretch of the passenger version with the same range as the 747-8I could be built using common parts, but I would still think it would not have that big a potential market compared to the development costs.
If the 777-200LR were to operate successfully on the LHR-SYD route for several years, Boeing will probably build a long ranged variant of the Y3 twin-engined replacement for the 777-300ER and 747 which will be built of materials used in the 787 and later projects.
I agree, Paleo, your reasoning makes a lot of sense. The 747-8 series derivatives seem to be more suited to freighters and demand is the key. If a customer wanted a -8 variant in sufficient quantity, that would be another story.