That is not entirely true - I cannot speak for NW, but UA use the 747 on SFO-FRA daily, soon on one SFO-LHR frequency, IAD-LHR and IAD-FRA. It is true that the equipment varies by season on some routes tho. UA sends about equal parts 747 and 777 to Asia - 74's to HK and beyond as well as one NRT, 777 to PEK, PVG, NRT, etc.

The economics of a route are what makes the 747 either profitable or not. Some routes can support the higher pax load and, certainly, a higher cargo payload. I have read anecdotally that UA's daily HKG-SGN-HKG flight is mostly cargo and the pax load tends to be very light. UA is the only US carrier that serves SGN and it's a one-stop flight with the same flt number and aircraft so this probably results in full loads of cargo going to and from Vietnam and the US. Some have even speculated that they sell fewer seats than are available on this route to save space for the money maker - cargo.