Which is what they have already started to do!
And if you are comparing BA service to that of KLM and EI there is no contest. Premium would stick with BA all the way unless there was a HUGE financial saving which at LHR are few and far between.
You were talking of 3rd runway.
Assumming BA and everybody else get slots in proportion to existing slots then AF/KLM, LH and everybody else will significantly increase number of slots that they have. Ok thats stating the obvious
If AF/KLM OR LH get new slots will they be used for inter european routes ?
Highly unlikely as simply not worth it and PAX numbers not going to grow by that much. Hence you have lots of airlines with slots which are not huge profit benefit by using on existing routes so they will look to expand other routes they are not serving from LHR.
100% premium service from LHR to US or other good BA premium routes starts to hit BA quite hard especially as you can afford to be generous as its all new and you are forcing BA prices significantly downwards.
There is a huge revenue risk to BA and BA premium service may be better but paying 40% of BA cost changes lots of peoples perspectives.