As I understand, airlines are limited to add additional flights at LHR as there is not enough slot capacity. The new terminal will of course not change the situation - but maybe attract even more passengers.
1.) The A380 would help an airline to grow as more passengers can be transported on a flight.
2.) Replacing e.g. 3 B747s with 2 A380s would free a slot. The slot could be used for new routes.