I agree BA should not move other passengers around to please frequent flyers, that said I must confess I am disappointed at how BA treats seats reassignments.
Indeed there is nothing wrong with moving the curtain or swapping planes but, for instance, if I booked an emergency seat on a A319 and then it gets changed to a A321, BA should retain the exit row selection and move me to the closest seat matching that requirement. Surely something that can be easily programmed. Instead, they either keep the same seat number or randomly allocate another seat.
This forces me to constantly monitor all my upcoming bookings to check my seats, also because BA does not bother to proactively send an email to let passengers know.
Overall, seat assignment is IMO very poorly managed.