I agree with your logic on why the 777-200ERs are not receiving the new F seat, but there are a four aircraft (G-YMMR/S/T/U) that are significantly younger than the 747s were at 11 years old (delivered in 2009) - I would expect those four to be around for another decade or so unless BA go down the fleet simplification route as they are likely to do with the A380s.