That is the other aspect, there becomes a point where air freight is just not a viable or efficient means of transporting goods.
Outside of the time sensitive scheduled parcel delivery companies which use pure freight and belly hold space, the bulk cargo which negates air transport quickly diminishes to very niche markets. I’m thinking things like imports of flowers, fish, any highly perishable product. Of course PPE during covid but I suppose the less said about that the better.
I’d question just to what extent there is a genuine scaleable market for air freight that cannot already be handled within current airport capacity. What is currently being transported by road rail or sea that would be better transported by air? It’s why when people start saying that airports near a railway line/shipping port are idea for freight. Why exactly? They come by ship and rail for a reason, typically because it’s bulk cargo that just cannot be delivered any other way and air transportation is simply impractical.
Broadly speaking there’s three types of product that comes in nh air:
1) parcels, including various low weight individual packages bought on Amazon etc with next day or 48h delivery. Most of that is handled by the usual parcels firms (DHL, UPS, etc.) and much of this is on flights run by or chartered by the parcels companies to EMA to get to their GT (Golden Triangle) hubs. There are exceptions of course - DHL have a daily flight or two into Luton for example, although i suspect that might suffer once the Amazon fulfilment centre at Hemel is closed.
2) high value, low weight palletised goods, typically technology eg laptops, iphones, etc. As often as not in the belly, so just as likely to appear at LHR as anywhere else. Onward distribution of these depends on the customer, but it will usually be via the retialers DC, many of which are in the GT.
3) Certain types of perishable food, usually from outside europe. Almost exclusively belly freight, and usually arrives LHR and distributed from there (albeit often via the retailers perishable DC, some of which are in the GT)