I doubt BA will pull the plug on its LON-AMS routes..seeing that it may have enough loyal flyers (Club or Oneworld) that adores T5 to connect & BA services. And also those who flies with them because of a better fare.
Maybe a reduction in frequentcy especially from LGW?
I don't think they want to give it all to U2

LGW is not that far away by train. If I compare both of the Express (Heathrow & Gatwick) service it's only 15 mins more.

BA & KLM are surely not saddened with BMI leaving and might secretly have a farewell party

And did you meant breakthrough with the word "doorstart"?