Dobbo, I'm not sure whether you are arguing for or against expansion at LHR but you provide another good reason for it.
Heathrow has, as you say created much growth and employment in a large area.
All those businesses you mention above pay tax. The jobs created attract income tax, the companies pay corporation tax and the services attract VAT.
So why is it so unthinkable that some of this tax goes towards expansion that would lead to more of that income in the future?