HD you are dead right. I wonder if there has been a day in the life of LHR since say 1960 when there wasnt a large (lets say equivalent to a 6 story office block or bigger) going on somewhere on the airport or failing that something akin to a pretty large piece of roadworks.
Its abi of an irony that the original LHR was one of the few UK Gov projects that started out with a huge amount of excess capcity for movements and a huge amount of development space for longer runways (the 09/27s) and terminals . Of course that couldnt last and so from some point, in the 80s, maybe expansion on a strategic scale, was wilfully ignored.
What no one in politics seems to understand is that airports are all about scale and connectivity and ideas like 'runways in the SE of England being effectively interchangeable is nonsense. No one would look at growth on the London to Manchester rail route and say lets build an extra line to Leeds-they are all in the North and can be substituted.
So once you have a big hub airport you have to expand it - improving capacity at a secondary airport is no use at all nor is building new airport miles from anywhere going to work because you have to move tens of thousands of people and build ultra expensive new rail links through congested suburbs to reach it causing as much disruptionsthe airport itself.