As Clipper already said, these aircraft serve different markets.
The 747-400 production line has slowed down in the past three years, because of changes in the World economy and the emerging new trends in air travel.
The A3XX may be a success in high-density routes, where the airports have slot restrictions, like Heathrow, Tokyo Narita and Frankfurt. Howevere, Boeing was not very enthusiastic in developing a similar jet. Is this a bluff? Well... Probably CEO Phil Condit is the only person who has the answer.
I also think that the 747-type is an inferior product. Not in terms of equipment quality, but in terms of service it aims to provide: Low-frequency schedules and hub-to-hub routes.
I believe the future trends will focus even more in business travelling. Executives demand more frequencies and more direct flights. This is why the 777 has been such a success!
Regarding a 777 freight version, I don't think it is economic viable. It is like buying a Mercedes passenger car to make pizza deliveries. The 747-400 are fine for cargo , because, as I already mentioned, it offers bulky service. The 747-400F's mile costs per kilo flown are low.
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