Agreed nor is it a god given right to expect it to be so.
Nobody says it is, hence the risk being almost completely taken on by the owners of LHR, which is another reason it is such a good deal for the UK.
And it has nothing to do with the primary reason for the commissioned report which was about once again RUNWAY capacity
The whole SE runway capacity argument came a long time after Heathrow announced plans to expand. The whole idea that you can build a runway anywhere in the SE no matter where was just a political escape route out of Heathrow expansion that had already been given the green light. The arguments for expanding LHR and LGW are separate cases and giving LGW a new runway doesn't change Heathrows expansion plans.
Remember that when the idea was first formed LHR and LGW were owned by the same company. They chose to expand/modernise LHR and have pretty much been doing so continually ever since.