The transit passengers fill say half the plane. The other half gets taken by O&D passengers from the local region. Neither flow would justify the density of flights on their own, but each supports the other and then there is an economic cascade process that rises from that, because the place gets so well connected internationally.
Exactly. I've never understood why some people appear unable to grasp what is, after all, a fairly simple concept.
Granted, hubbing isn't universally popular - most passengers would prefer to fly direct, if such a service existed, rather than via a hub, but the point is that in many cases there probably won't be the demand to make a direct flight viable.
While some of Heathrow's current
PR campaign is dire, bordering on disingenuous, they have actually made a rather good video explaining the dynamics of hubs, which should be required viewing for contributors to the debate:
Apart from the inexplicable reference to some place called "Dyce", wherever that is (it's actually the original name of what is now Aberdeen Airport).
Of course nothing in the video is specific to Heathrow, it would apply to UK plc's hub wherever that ends up being located.