Aviation is never going away. The Govt should've steamrolled over everything and everyone and allowed the building of this third runway over a decade ago with T5 (even help fund it).
Heathrow should be building more satellites for T5, Knocking down T1 to expand T2, replacing T3 with a modern design and building a new terminal between the new and existing runways.
It's as much about future-proofing the place as it is expanding. A massive hub airport to dominate Europe would be good for the economy.