The 747 was only on most US routes because it had a lot of business class seats (when refitted), fair cargo space, and had a low total operation cost on a short route (the aircraft being fully paid and the increased fuel consumption being less significant on a short route). So it could be replaced by a 777, 787, or A350 in most cases. It's not like passenger loads will be back up to previous levels (especially in premium cabins on business routes) anytime soon.

The exception, I believe, is Phoenix where the 747 had better performance margins on departure in the hot-and-high environment, enabling a good cargo load as well as passengers. Jo'burg can have the same issue.

Other routes could be handled by a 777 or an A350. Maybe PHX can be handled by the A350, even (I'd love to hear comments on this from an actual BA pilot or engineer).