Well I flew this week from LHR-EWR on a 777 (G-RAES) and it has one of the best economy class products on the market IMO. The 747s are already starting to be phased out and BA use the 77W on some of the LAX runs.
And to answer your question about putting the 747 on the route it is for the greater capacity the 747 has i.e. 70 in J